Process for production of magnesium fluoride sol solutions from alkoxides comprising addition of magnesium salts

ABSTRACT

The invention relates to a method for obtaining a magnesium fluoride (MgF2) sol solution, comprising the steps of providing a magnesium alkoxide precursor in a non-aqueous solvent and adding 1.85 to 2.05 molar equivalents of non-aqueous hydrofluoric acid, characterized in that the reaction proceeds in the presence of a second magnesium fluoride precursor selected from the group of salts of strong, volatile acids, such as a chloride, bromide, iodide, nitrate or triflate of magnesium, or of a catalytic amount of a strong, volatile acid; and/or an additive non-magnesium fluoride precursor selected from the group of salts of strong, volatile acids, such as a chloride, bromide, iodide, nitrate or triflate of lithium, antimony, tin calcium, strontium, barium, aluminium, silicium, zirconium, titanium or zinc. The invention further relates to sol solutions, method of applying the sol solutions of the invention to surfaces as a coating, and to antireflective coatings obtained thereby.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 14/428,989,filed Mar. 18, 2015, issued as U.S. Pat. No. 10,081,004 on Sep. 25,2018, which is the US National Stage of PCT/EP2013/069412, filed Sep.18, 2013, which claimed the benefit of European Patent Application12184916.0, filed Sep. 18, 2012.

FIELD

The present invention relates processes for the synthesis of MgF₂ solsfrom magnesium alkoxides, and methods for manufacturing optically activesurface coatings comprising MgF₂ as optically active component. Theinvention also encompasses solar panels, architectural glass, opticalsystems and lenses coated by the surface coatings of the invention.

BACKGROUND

The reduction of light reflection is desirable in many applications suchas photovoltaic and photothermic elements, architectural glass oroptical elements. The reflection of visible light passing through anoptically transparent substrate (e.g., glass) can be reduced by coatingthe substrate with an optical active thin layer exhibiting a refractiveindex laying between the refractive index of the substrate (n_(S)˜1.5 incase of glass) and that of air (n_(air)=1). An ideal single layercoating material would have a refractive index around n_(c)˜1.23,resulting in a nearly 100% optically transparent system (see FIG. 1).

Multi-layer systems (interference layers, usually alternating layers ofhigh reflective TiO₂- and low reflective SiO₂-films) are known in theart. They suffer from high production costs and complex methods ofmanufacture.

Anti-reflective (AR) oxide monolayers are known in the art. The oxidematerial with the lowest refractive index is SiO₂ (n_(SiO2)˜1.46). Toachieve lower n values, porosity has to be introduced into such layers.Up to more than 50% porosity, however, is needed to provide porouslayers with n=1.23, resulting in low mechanic stability of such layers.

Some metal fluorides exhibit refractive indexes significantly lower thanSiO₂. Magnesium fluoride is the most investigated material(n_(MgF2)=1.38). Other properties such as scratch resistance, mechanicstability, thermal stability and hydrolysis resistance are important forapplications such as coatings of glass or polymers.

Vapor phase deposition methods such as sputtering can be employed forfilm deposition. Thin layers obtained thereby do usually not showsignificant porosity, resulting in a refractive index of close to orcorresponding to that of the bulk material. Vapor phase depositedMgF₂-layers suffer from non-stoichiometric adjustment of metal tofluorine ratios, leading to point defects (“F-center”) formation,resulting in impaired optical quality of the layer. Evaporation methodsmay aid in overcoming this problem, however large area depositions arenot facilitated thereby because of point defect formation. Theintroduction of the necessary porosity is insufficient in both methods.

Metal fluoride layers generated from liquid phase deposition weredescribed first based on metal trifluoroacetate sols (S. Fujihara, inHandbook of Sol-Gel Science and Technology, ed. S. Sakka, Kluwer,Boston, 2005, vol. 1, pp. 203-224). In a first step, the metal fluoridetrifluoroacetates are deposited onto the substrate and are subsequentlydecomposed thermally, resulting in very porous metal fluoride layers.Due to the formation of hydrogen fluoride during this thermaldecomposition process and a drastic shrinking of the layer thickness, anadjustment of the parameters of such layers is difficult. Moreover, thecoated substrate as well as the equipment can undergo corrosion causedby evaporated hydrogen fluoride gas. Insufficient mechanical performanceof the resulting layer is a further drawback of this technology.

U.S. Pat. No. 6,880,602B2 (EP 1 315 005 B1) shows a process forobtaining sol solutions of magnesium fluoride by reacting magnesiumacetate or methoxide with aqueous hydrofluoric acid in methanol atelevated temperatures under high pressure. This process suffers fromsignificant disadvantages when applied in technical scale, such as theneed for high pressure batch reactions and the use of methanol.

US 2011/0122497 A1 (EP 1 791 002 A1) shows MgF₂-sols obtained by a highpressure process, with added SiO₂-sols as “binders”, which results inacceptable optical and mechanical characteristics of the layers.

Clear magnesium fluoride based sol solutions that are suitable forcoating glasses have not been accessible so far from alkoxides otherthan from magnesium methoxide. Furthermore, the methoxide has only beenaccessible in methanol, a solvent that poses problems related to itstoxicity and workplace safety profile. Magnesium methoxide is notsoluble in ethanol or isopropanol, thus blocking the path to clear solsolutions. Magnesium ethoxide, which is available commercially at lowerprices, does not dissolve directly in methanol, ethanol or isopropanol.

SUMMARY

The present invention aims at the preparation of low refractive indexantireflective layers that overcome the drawbacks of the state of theart, particularly in providing a general pathway to obtain clear solsolutions suitable for antireflective coatings, from magnesiumalkoxides, particularly magnesium methoxide or ethoxide. This problem issolved by the methods, preparations and coatings as defined by theindependent claims.

SHORT DESCRIPTION OF THE FIGURES

FIG. 1 shows the change in the optical transmittance of a coatedmaterial as a function of the refractive index of the thin film;

FIG. 2 shows a typical MgF₂-particle size distribution determined byDLS;

FIG. 3 shows a typical XRD-diffractogram of a MgF₂-Xerogel obtained froma magnesium ethylate based MgF₂-Sol;

FIG. 4 shows a typical ¹⁹F-NMR-spectrum during the synthesis of a MgF₂sol indicating the stage of completeness of MgF₂-formation (the chemicalshift at −198 ppm represents pure MgF₂, and only the d and e graphscorrespond to stoichiometric near ratios (d:F/M=1.95 and e:F/Mg=2.0));

FIG. 5 ¹⁹F NMR spectra of MgF₂-sols prepared from Mg(OC₂H₅) and MgCl₂(prepared according to the general synthesis described in example 8);

FIG. 6 ¹⁹F NMR spectra of MgF₂-sols prepared from Mg(OC₂H₅) plusMgCl₂.6H₂O (prepared according to the general synthesis described inexample 9).

DETAILED DESCRIPTION

According to a first aspect of the invention, a method for obtaining amagnesium fluoride (MgF₂) sol solution is provided, comprising the stepsof

-   a. providing a magnesium alkoxide (Mg(OR)₂) in a non-aqueous solvent    in a first volume and-   b. adding, in a second volume, 1.85 to 2.05 molar equivalents of    anhydrous hydrogen fluoride (HF) to said magnesium alkoxide,    -   characterized in that-   c. the reaction proceeds in the presence of    -   i. a second magnesium fluoride precursor selected from the group        of salts of strong, volatile acids, such as chloride, bromide,        iodide, nitrate or triflate of magnesium and/or    -   ii. at least one additive non-magnesium fluoride precursor        selected from the group of salts of strong, volatile acids, such        as chloride, bromide, iodide, nitrate or triflate of of lithium,        antimony, tin, calcium, strontium, barium, aluminium, silicium,        zirconium, titanium and/or zinc.

In some embodiments, said reaction proceeds in the presence of of acatalytic amount of a strong, volatile acid and

-   -   i. a second magnesium fluoride precursor selected from the group        of salts of strong, volatile acids, such as chloride, bromide,        iodide, nitrate or triflate of magnesium and/or    -   ii. at least one additive non-magnesium fluoride precursor        selected from the group of salts of strong, volatile acids, such        as chloride, bromide, iodide, nitrate or triflate of of lithium,        antimony, tin, calcium, strontium, barium, aluminium, silicium,        zirconium, titanium and/or zinc.

In some embodiments, the magnesium alkoxide is magnesium methylate(Mg(OMe)₂), magnesium ethylate (Mg(OEt)₂), magnesium propylate(Mg(OPr)₂) or magnesium butylate (Mg(OBu)₂), in particular magnesiumethylate and magnesium methylate.

In some embodiments, the magnesium alkoxide is magnesium ethylate(Mg(OEt)₂), magnesium propylate (Mg(OPr)₂) or magnesium butylate(Mg(OBu)₂), in particular magnesium ethylate.

According to a second aspect of the invention, a method for obtaining amagnesium fluoride (MgF₂) sol solution is provided, comprising the stepsof

-   -   a. providing a magnesium ethanolate (Mg(OEt)₂), propylate        (Mg(OPr)₂) or buthylate (Mg(OBu)₂), in particular a magnesium        ethanolate, in a non-aqueous solvent, in particular in ethanol        or isopropanol, in a first volume and    -   b. adding, in a second volume, 1.85 to 2.05 molar equivalents of        anhydrous hydrogen fluoride (HF) to said magnesium alkoxide,        -   characterized in that    -   c. the reaction proceeds in the presence of a catalytic amount        of a strong, volatile acid.

In some embodiments, said method for obtaining a magnesium fluoride(MgF₂) sol solution comprises the steps of

-   -   a. providing a magnesium ethanolate, in a non-aqueous solvent,        in particular in ethanol or isopropanol, in a first volume and    -   b. adding, in a second volume, 1.85 to 2.05 molar equivalents of        anhydrous hydrogen fluoride (HF) to said magnesium alkoxide,        -   characterized in that    -   c. the reaction proceeds in the presence of a catalytic amount        of a strong, volatile acid, and        -   i. a second magnesium fluoride precursor selected from the            group of salts of strong, volatile acids, such as a salt of            a chloride, bromide, iodide, nitrate or triflate of            magnesium, and/or        -   ii. at least one additive non-magnesium fluoride precursor            selected from the group of salts of strong, volatile acids,            such as a salt of a chloride, bromide, iodide, nitrate or            triflate of of lithium, antimony, tin, calcium, strontium,            barium, aluminium, silicium, zirconium, titanium or zinc.

If not stated otherwise, the following embodiments and descriptionsrefer to the first and the second aspect of the invention.

The inventors have surprisingly found that addition of a secondmagnesium fluoride precursor and/or an additive non-magnesium fluorideprecursor to the Mg-alkoxide solvent system facilitates the sol reactionand improves the optical and mechanical properties of the coating layersobtained from the resultant sols. The coatings derived thereof showincreased amorphicity and porosity.

As used herein, the term “a second magnesium fluoride precursor” (alsoreferred to as “second magnesium precursor”) refers to an additionalmagnesium compound present in the fluorination reaction. Depending onthe reaction conditions, this compound may be completely fluorinated tomagnesium fluoride, partially fluorinated or not fluorinated at all.

As used herein, the term “non-magnesium fluoride precursor” (alsoreferred to as non-magnesium precursor) refers to an additional metalcompound present in the fluorination reaction. Depending on the reactionconditions this compound may be completely fluorinated to magnesiumfluoride, partially fluorinated or not fluorinated at all.

The second magnesium fluoride precursor and the non-magnesium fluorideprecursor are both added additionally in a certain amount to themagnesium alkoxide present in the fluorination reaction. Both are alsoreferred to as “additives”, “metal additive” or “additive precursors”.

In some embodiments, in addition to the before mentioned magnesiumadditive compound a further metal additive compound is present incertain amounts (see discussion below). In some embodiments, in additionto the before mentioned magnesium additive compound further magnesiumadditive compounds are present. In some embodiments, two or more metaladditive compounds are present (see discussion below). In someembodiments, two or more metal additive compounds and two or moremagnesium additive compounds are present. However, the sum of molarratio of the magnesium additive(s) and the further metal additive(s)does not exceed an amount of 0.2 mole calculated per mole magnesium withrespect to the first magnesium precursor.

In some embodiments, the salts of strong, volatile acids are selectedfrom chloride, bromide, iodide, nitrate or triflate.

Addition of a Second Magnesium Fluoride Precursor:

The chemical principle behind the advantages conferred by addition ofthe additive(s) mentioned above appears to be the cleavage of theMg—O—Mg-bonds that can not be attacked by HF under the fluorolyticsol-gel conditions. Addition of a metal salt of a strong acid such asHCl, e.g. MgCl₂, MgBr₂, MgI₂ or Mg(NO₃)₂ in small amounts (1 partchloride/halogenide/nitrate to 10-40 parts MgOR₂, or 1 partchloride/halogenide/nitrate to 5-20 parts MgOR₂) to the turbidsuspension of the magnesium alkoxide leads to the formation of clearalkoxide solutions after HF-addition.

Technical magnesium alkoxides are always hydrolysed to a certain degree,leading to Mg—O—Mg-units being present in the material. These Mg—O—Mgunits do not undergo reaction with HF under the conditions of thereaction (in dry solvents), thus preventing the formation of clearMgF₂-sols. Without wishing to be bound by theory, the inventors believethat upon addition of HF to a mixture of alkoxide and chloride, HFreacts preferentially with the chloride (or the salt of another strongacid), leading to release of HCl (or the other strong acid). The HCl orother acid then breaks the Mg—O—Mg-units. As a result, complete reactioncan take place, resulting in the formation of clear MgF₂-sols.

In some embodiments, the molar amount of the second magnesium fluorideprecursor is 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

In some embodiments, the second magnesium precursor (also referred to asan metal additive precursor) is present in an amount of

-   -   0.2 mole (1:5) to 0.001 mole (1:1000) per mole magnesium,    -   0.2 mole (1:5) to 0.01 mole (1:100) per mole magnesium,    -   0.2 mole (1:5) to 0.04 mole (1:25) per mole magnesium,    -   0.2 mole (1:5) to 0.08 mole (1:12.5) per mole magnesium,    -   0.1 mole (1:10) to 0.01 mole (1:100) per mole magnesium,    -   0.1 mole (1:10) to 0.04 mole (1:25) per mole magnesium, or    -   0.01 mole (1:100) to 0.04 mole (1:25) per mole magnesium

with respect to the magnesium alkoxide.

In some embodiments, the second magnesium fluoride precursor is selectedfrom the group of chloride, bromide, iodide, nitrate or triflate, inparticular from chloride or nitrate, more particularly from chloride.

In some embodiments, in addition to the before mentioned magnesiumadditive compound a further metal additive compound is present incertain amounts (see discussion below). However, the sum of molar ratioof the magnesium additive and the further metal additive does not exceedan amount of 0.2 mole calculated per mole magnesium with respect to thefirst magnesium precursor.

In some embodiments, the second magnesium fluoride precursor is MgCl₂.In one embodiment, the molar amount of MgCl₂ in relation to the amountof magnesium alkoxide is 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%.

In some embodiments, the same result is obtained by adding a catalyticamount, such as 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%, of a strong,volatile acid such as HCl-gas or concentrated hydrochloric acid (in thecase of addition of concentrated acid in water, the restriction of watercontent of the reaction must be taken into account, see below).Different volatile acids can be employed instead of HCl, such as HBr,HI, HNO₃ and the like. The requirement for volatility is based on thesubsequent use of the fluoride sol solution in formation of an opticallayer. The volatility of the acid guarantees that the acid will beeasily removed and not impact the optical, mechanical or chemicalproperties of the layer.

In some embodiments, trifluoroacetic acid (TFA) is added to the firstvolume or the second volume. The use of small amounts of TFA leads toshorter stirring times. By using TFA, the sol solutions clear faster.The inventor believes, without wishing to be bound by theory, thatagglomeration products comprising a small amount of HF within theagglomeration products are generated intermediately. Thus, some parts ofthe magnesium precursor have not yet reacted with HF in a first phase ofthe reaction. A complete reaction with HF occurs when the HF diffusesinto the agglomeration products and reacts with the remaining magnesiumprecursor, which leads to a clear sol solution. By adding small amountsof TFA, the TFA can react with the remaining magnesium precursoryielding a clear sol solution. The TFA-magnesium-intermediate can betransformed by heating (as will be described later) to MgF₂. If metaladditives (in comparison to the reaction of only magnesium precursors)are additionally used, even less TFA is necessary in order to obtain aclear sol solution in a reasonable amount of time.

The magnesium salts (bromides, iodides, nitrates etc.) offer theadvantage of leading to a similar degree of resolvation of the otherwiseinsoluble alkoxide while avoiding the technical drawbacks of handlingliquid acids or their respective gases.

Addition of an Additive Non-Magnesium Fluoride Precursor:

Chlorides, bromides, iodides or other suitable salts of strong acids ofmetals different from magnesium, such as—by way of non-limitingexample—lithium, antimony, tin, calcium, strontium, barium, aluminium,silicium, titanium, zirconium, or zink, may also be used, leading toadditional synergistic effects both regarding the stability of the solsolution and the mechanical and optical qualities of the resultantlayers.

In some embodiments, an additive non-magnesium fluoride precursorselected from the group of salts of strong, volatile acids, inparticular from chloride, bromide, iodide, nitrate or triflate oflithium, antimony, tin, calcium, strontium, barium, aluminium, silicium,zirconium, titanium or zinc is present in the first volume.

In some embodiments, an additive non-magnesium fluoride precursorselected from the group salts of strong, volatile acids, in particularfrom chloride, bromide, iodide, nitrate or triflate of lithium,antimony, tin, calcium, strontium, barium, aluminium, silicium,zirconium, titanium or zinc is is added after step b (the addition ofthe second volume).

The major advantage conferred by these additives is the faster clearingof the sols, their better stability against gelation and the improvedmechanic and thermal stability of the layers obtained from such sols.

In some embodiments, the metal of the non magnesium fluoride precursoris Li⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sn²⁺, Zn²⁺, Al³⁺, Si⁴⁺, Ti⁴⁺, Zr⁴⁺, Sn⁴⁺, Sb³⁺or Sb⁵⁺, in particular Li⁺, Ca²⁺, Ti⁴⁺, Al³⁺ or Si⁴⁺.

In some embodiments, the at least one additive non-magnesium precursoris a chloride or a nitrate, in particular a chloride.

In some embodiments, the at least one additive non-magnesium precursoris a chloride, bromide or nitrate of lithium, antimony, tin, calcium,strontium, barium, aluminium, silicium, zirconium, titanium or zinc, inparticular of lithium, calcium, silicium, titanium or aluminium, moreparticularly of calcium. In some embodiments, the at least one additivenon-magnesium precursor is a chloride of lithium, calcium, silicium,titanium or aluminium.

In some embodiments, the non-magnesium precursor (also referred to asthe metal additive precursor) is present in an amount of

-   -   0.2 mole (1:5) to 0.001 mole (1:1000) per mole magnesium,    -   0.2 mole (1:5) to 0.01 mole (1:100) per mole magnesium,    -   0.2 mole (1:5) to 0.04 mole (1:25) per mole magnesium,    -   0.2 mole (1:5) to 0.08 mole (1:12.5) per mole magnesium,    -   0.1 mole (1:10) to 0.01 mole (1:100) per mole magnesium,    -   0.1 mole (1:10) to 0.04 mole (1:25) per mole magnesium, or    -   0.01 mole (1:100) to 0.04 mole (1:25) per mole magnesium

with respect to the magnesium alkoxide.

In some embodiments, the amount of additive non-magnesium fluorideprecursor is 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 12%, 14%, 16%, 18%or 20% of the magnesium alkoxide, as measured in molar equivalents.

The major advantage conferred by these additives is the faster clearingof the sols, their better stability against gelation and the improvedmechanic and thermal stability of the layers obtained from such sols.

In some embodiments, the additive non-magnesium fluoride precursor isCaCl₂. In one embodiment, the molar amount of CaCl₂ in relation to theamount of magnesium alkoxide is 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%,12%, 14%, 16%, 18% or 20%.

In some embodiments, the second metal fluoride precursor is Ca²⁺, Sr²⁺,Ba²⁺, Al³⁺, Si⁴⁺, Ti⁴⁺, Zr⁴⁺ or Zn²⁺.

The MgF₂ sols of the invention are synthesized by reacting a solution ofa suitable magnesium alkoxide, such as—by way of non-limitingexample—magnesium methanolate (methylate), ethanolate (ethylate) orpropanolate, in organic water-free solvents like alkohols, polyalcohols,ethers, esters, and mixtures thereof with anhydrous hydrogen fluoride(HF) in the presence a second magnesium fluoride precursor and/or anon-magnesium fluoride precursor. The HF is provided in gaseous or inliquid form or as solution in a solvent as aforementioned. In oneembodiment, the non-aqueous solvent is ethanol or isopropanol, inparticular ethanol.

In one embodiment, the non-aqueous solvent is ethanol or isopropanol, inparticular ethanol, and the precursor is magnesium ethylate.

Addition of Carbon Dioxide:

Additionally, carbon dioxide may be added to the reaction, furtherimproving the solubility of the alkoxide precursor and the stability ofthe resultant sol solution. Again without wishing to be bound by theory,the inventors believe that the surprising advantage of using carbondioxide derive from the fact that the addition of CO₂ to the suspension,particularly to an alkoholic suspension, causes a reaction of the acidicgas CO₂ with the basic Mg—O-bonds, resulting in the partial formation ofan intermediate that might be formulated as a kind of magnesiumalkylcarbonate, which under the invented circumstances is soluble,particularly in alcohols.

The same mechanism also applies to the intact magnesium alkoxide inthose cases where the alkoxide is unsoluble in the respective solvent.One example of such pair is Mg ethoxide in ethanol. As a consequence,the insoluble magnesium ethoxide becomes converted not only into asoluble intermediate precursor but also becomes activated by formationof the above mentioned alkyl carbonate to undergo reaction with HF, thusleading to formation of clear MgF₂-sols.

Any imaginable addition of CO₂ is possible: In some embodiments, CO₂ isadded as a gas. In some embodiments, CO₂ is added as a solid (dry ice).In some embodiments, CO₂ is added prior to the addition of HF. In someembodiments, CO₂ is added prior to the addition of HF and addition ofCO₂ is continued while HF is added.

The temperature of the reaction in presence of CO₂ might be varied overa wide range from minus 60° C. up to at least +50° C. In any case clearsolutions of the magnesium alkoxide, particularly ethoxide, will beobtained. The time until the suspension clears up will differ dependingon the conditions. The CO₂-content may range widely, for example from 1to 7 mass % CO₂ in relation to ethanol, or from 2 to 4%.

In some embodiments, gaseous carbon dioxide is carried through the firstvolume prior to adding the second volume comprising anhydrous hydrogenfluoride, thus, prior to the addition of HF In one embodiment, theamount of carbon dioxide is adjusted in order for the first volume toreach a CO₂ content of between 1% and 5% (w/w), in particular to reach aCO₂ content of 3% (w/w). In one embodiment, the amount of carbon dioxideis adjusted in order for the first volume to reach a CO₂ content ofbetween 1% and 4% (w/w).

In some embodiments, carbon dioxide is added to a dispersion ofmagnesium ethanolate in ethanol in a range between 1 to 4 Vol % in atemperature interval between −60° C. to +50° C. In some embodiments, thetemperature selected is between 10° C. and 25° C. In some embodiments,carbon dioxide is added to a dispersion of magnesium ethanolate inethanol in a range between 1% and 5% (w/w), in particular of 3% (w/w) ina temperature interval between −60° C. to +50° C. In some embodiments,the temperature selected is between 10° C. and 25° C.

In some embodiments, said magnesium alkoxide is partially hydrolysed,comprising Mg—O—Mg bonds. In some embodiments, said magnesium alkoxideis technical grade magnesium alkoxide. In some embodiments, saidmagnesium alkoxide is technical grade magnesium ethanolate.

The MgF₂ sols of the invention are synthesized by reacting a solution ofa suitable magnesium alkoxide, such as—by way of non-limitingexample—magnesium methanolate (methylate), ethanolate (ethylate) orpropanolate, in organic water-free solvents like alkohols, polyalcohols,ethers, esters, and mixtures thereof with anhydrous hydrogen fluoride(HF) in the presence of carbon dioxide, a second magnesium fluorideprecursor and/or a non-magnesium fluoride precursor. The HF is providedin gaseous or in liquid form or as solution in a solvent asaforementioned.

The method according to the invention differs from methods known in theart by carefully controlling the water content of the reaction volume(i.e., the water content of educt and its solvent, the water content ofthe HF added, and the amount of water resulting from the reaction andfurther reactions downstream, for example, from a reaction betweenadditive ligand, and the solvent.

In some embodiments, the magnesium alkoxide is magnesium methoxide ormagnesium ethoxide, and the reaction solvent is ethanol. Therein, thewater content of the final sol solution is determined by the watercontent of the solvent, the water content of the magnesium alkoxide andthe water content of the HF solution (if a solution is used, as ispractical in order to adjust the amount of HF to an exact stoichiometricequivalent).

The method according to the invention does not rely on complete absenceof water. The solvents used for preparing the first and second volume(in embodiments where HF is applied in solution) of the reaction do nothave to be dried. In practice, the use of “absolute” solvents assupplied technically, having a water content of equal to or smaller than0.2% (w/w) has been sufficient. Magnesium alkoxides such as magnesiummethanolate or ethanolate were used as commercially obtained.

In some embodiments, the water content of the sol solution is equal toor lower than 2.8, 2.7, 2.6, 2.5, 2.4, 2.3, 2.2, 2.1, 2.0, 1.9, 1.8,1.7, 1.6, 1.5, 1.4, 1.3, 1.2, 1.1, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4,0.3, 0.2 or 0.1 mole water per mole magnesium. One limit observed in theexperiments leading to the present invention was that the reaction mustnot exceed a water content of 3% (volume). In some embodiments, thewater content of the first volume is equal to or lower than 3%, 2.75%,2.5%, 2.25%, 2.0%, 1.75%, 1.5%, 1.25%, 1.0%, 0.75%, 0.5%, 0.25% or 0.1%.

In some embodiments, step b. is performed under vigorous stirring. Insome embodiments, the stirring speed exceeds 100 rpm, 150 rpm, 200 rpm,250 rpm or 300 rpm. In some embodiments, the stirring speed is in therange of 100 rpm to 1000 rpm, in particular 600 rpm to 1000 rpm. In someembodiments, the stirring speed exceeds 100 rpm, 200 rpm, 300 rpm, 400rpm, 500 rpm, 600 rpm, 700 rpm, 800 rpm, 900 rpm or 1000 rpm, inparticular the stirring speed exceeds 600 rpm, 700 rpm, 800 rpm, 900 rpmor 1000 rpm.

In some embodiments, the reaction volume resulting from the secondvolume being added to the first volume is stirred for 2 to 10 days, inparticular for 2, 3, 4, 5, 6, 7, 8, 9 or 10 days. In some embodiments,the reaction volume resulting from the second volume being added to thefirst volume is stirred for 8 to 20 hours, in particular for 8, 12, 16or 20 hours or 1 to 21 days, in particular for 1, 2, 3, 4, 5, 6, 7, 8,9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 days. The stirringtime depends on the used reactants and conditions, wherein the stirringwill be stopped after a clear sol solution is achieved.

One important parameter of the method according to the invention is theamount of HF being applied, the ideal amount being an exactstoichiometric equivalent (2 HF per Mg). Smaller amounts of HF willimprove sol formation, however will lead to impaired mechanical andoptical properties of the resulting coating. Larger amounts of HF leadto less stable sols or difficulties in sol formation. In general,amounts of HF exceeding 2 molar equivalents tend to favour crystallinerather than nanodisperse phases, leading to larger particle sizes and,eventually, precipitation.

In some embodiments, the amount of HF employed ranges from 1.9 mole to2.1 mole HF per mole Mg. In some embodiments, the amount of HF employedranges from 1.95 mole to 2.05 mole HF per mole Mg. In some embodiments,the amount of HF employed ranges from 1.98 mole to 2.02 mole HF per moleMg. In some embodiments, the amount of HF employed ranges from 1.99 moleto 2.01 mole HF per mole Mg.

For embodiments employing the use of an additive non-magnesium fluorideprecursor XB_(n), and where providing sol solutions for optical surfacecoatings is the objective, the acid BH formed by the reactionXB_(n) +nHF→XF_(n) +nBH

must be removed from the coating in order not to degrade its opticalproperties. The removal of the acid is easily effected by evaporationduring the drying or first thermal treatment of the coating. Hence, forembodiments of the invention where removal of the acid formed from themagnesium precursors is essential, the magnesium precursor is themagnesium salt of an acid that is volatile at the conditions employedduring drying and tempering, for example a precursor that is the salt ofan acid that is volatile (has a vapour pressure allowing the removal ofessentially all (>99%) of the acid from a 0.5 μm optical coating atambient pressure) at a temperature of 30° C., 40° C., 50° C., 60° C.,70° C., 80° C., 90° C., 100° C., 125° C., 150° C., 175° C., 200° C.,225° C., 250° C., 275° C. or 300° C.

In some embodiments a temperature in the range of about 100° C. to 500°C., in particular of about 250° C. to 500° C. can be used. In someembodiments a temperature in the range of about 400° C. to 500° C. canbe used. In some embodiments a temperature in the range of about 100° C.to 700° C. is used.

In some embodiments an additional amount of hydrogen fluoride (n_(adHF))is present in the fluorination of step b computed according to theformulan _(adHF)=(n _(M)*X_(additive))*Ox*A, wherein

-   -   n_(M) is the molar amount of said magnesium alkoxide,    -   X_(additive) is the molar percentage of said second magnesium        fluoride precursor or said non-magnesium fluoride precursor,        wherein        -   X_(additive) is in the range of 1% to 20%, and        -   Ox is the oxidation state of the metal of said second            magnesium fluoride precursor or said non-magnesium fluoride            precursor, and        -   A is selected from 0≤A≤1.

In some embodiments, a magnesium fluoride precursor and a non-magnesiumfluoride precursor may be present in the fluorination reaction. In someembodiments two non-magnesium fluoride precursors may be present in thefluorination reaction. The amount of additional HF is calculated asdiscussed above for each additional precursor. For example if magnesiumchloride and calcium chloride are present in the reaction the additionalamount of HF is calculated for the magnesium chloride and the calciumchloride, respectively.

In some embodiments two non-magnesium fluoride precursors may be presentin the fluorination reaction, wherein one of the two non-magnesiumfluoride precursors is selected from the group of salts of strong,volatile acids, such as—by way of non-limiting example—a chloride,bromide, iodide, nitrate or triflate, in particular chloride or nitrate,and the other non-magnesium fluoride precursors is selected from thegroup comprising an alcoholate (RO—), a carboxylate (RCOO—), acarbonate, an alkoxycarbonate, a hydroxide or a salt selected from thegroup of salts of strong, volatile acids, such as—by way of non-limitingexample—chloride, bromide, iodide, nitrate or triflate, in particularfrom alcoholate, nitrate or chloride.

In some embodiments a second magnesium fluoride precursor and anon-magnesium fluoride precursors may be present in the fluorinationreaction, wherein the second magnesium fluoride precursors is selectedfrom the group of salts of strong, volatile acids, such as—by way ofnon-limiting example—chloride, bromide, iodide, nitrate or triflate, inparticular chloride or nitrate, and the non-magnesium fluorideprecursors is selected from the group comprising an alcoholate (RO—), acarboxylate (RCOO—), a carbonate, an alkoxycarbonate, a hydroxide or asalt selected from the group of salts of strong, volatile acids, suchas—by way of non-limiting example—chloride, bromide, iodide, nitrate ortriflate, in particular from alcoholate, nitrate or chloride.

In some embodiments a second magnesium fluoride precursor and a furthermagnesium fluoride precursors may be present in the fluorinationreaction, wherein the second magnesium fluoride precursors is selectedfrom the group of salts of strong, volatile acids, such as—by way ofnon-limiting example—chloride, bromide, iodide, nitrate or triflate, inparticular chloride or nitrate, and the further magnesium fluorideprecursors is selected from the group comprising an alcoholate (RO—), acarboxylate (RCOO—), a carbonate, an alkoxycarbonate, a hydroxide or asalt selected from the group of salts of strong, volatile acids, suchas—by way of non-limiting example—chloride, bromide, iodide, nitrate ortriflate in particular from an alcoholate, nitrate or chloride, inparticular from alcoholate, nitrate or chloride.

For the embodiment making use of non-magnesium fluoride precursors, theresulting additive fluoride particles are, to the extent that theinventors have been able to characterize these phases, may not be doublesalts of magnesium and additive, but rather distinct species, present inthe sol as a mixture of single components (e.g. MgF₂ and the MF_(x)additive). In some embodiments, the additive fluoride particles have adiameter size of smaller than (<) 100 nm, <75 nm, <50 nm, <40 nm, <30nm, <20 nm or <10 nm. However, depending on the reaction conditionsresulting additive fluoride particles may be double salts of magnesiumand additive present in the sol as a mixture of double salt components.

In some embodiments, an amount of 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8% or 10%(mol equ.) of tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS) isadded to the magnesium precursor solution or suspension, leading to morerapid sol formation than without the additive.

Addition of said non-magnesium fluoride precursors promotes theformation of mostly amorphous nano-MgF₂ particles that are even smallerthan those obtained without non-magnesium fluoride precursors; thusresulting in an improved architectural ordering of the layer, and henceimproved mechanical strength. Based on ¹⁹F-NMR spectra (cf. FIG. 2),characteristic patterns for the MgF₂ particles synthesized this way canbe obtained, thus allowing distinction from MgF₂-materials obtainedaccording different syntheses approaches.

If metal M^(n+)-additives (derived from the second magnesium precursoror the non-magnesium precursor) are used, the HF-stoichiometry relatedto the first Mg²⁺-precursor (n_(HF)/n_(Mg2+)) can be varied between 1.6up to 2.2. In some embodiments, the stoichiometric ratio of HF tomagnesium precursor (n_(HF)/n_(Mg2+)) can be varied between 1.85 up to2.05, in particular between 1.9 up to 2.05. In some embodiments, the HFstoichiometry will be fixed to exactly 2.0 in reference to the magnesiumcontent of the first magnesium precursor. As long as stoichiometricamounts of HF are used (n_(HF)/n_(Mg2+)=2) formation of the other MF_(n)(M^(n+)=Li⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sn²⁺, Zn²⁺, Al³⁺, Si⁴⁺, Ti⁴⁺, Zr⁴⁺,Sn⁴⁺, Sb³⁺ or Sb⁵⁺) is suppressed as evidenced by NMR. In someembodiments, the HF stoichiometry will be fixed to exactly 2.0 inreference to the content of magnesium and additive, wherein the amountof fluoride that can react with the additive is computed according tothe oxidation state of the additive.

Thus, for embodiments where magnesium precursors and additives accordingto the above definition are present and an exactly stoichiometric amountof 2 eq HF per magnesium with respect to the first magnesium precursoris used, only the first magnesium precursor will be fluorinated. Theterm “only the magnesium precursor will be fluorinated” refers to anessentially quantitative conversion of the magnesium precursor. It ispossible that in some embodiments, traces of the magnesium precursorwill not be fluorinated and traces of the additive can be partially ortotally fluorinated, which may be due to an equilibrium in the solreaction. However, these traces will be in the range off around 1 to 2mol % related to MgF₂.

In some embodiments 2 eq HF are used and additionally a metal additiveis added to the sol reaction without a further addition of HF Thisallows a reaction of the metal additive with the remaining not reactedHF, as discussed above, yielding some partially fluorinated metaladditives while removing the unreacted HF from the reaction mixture. Ithas to be noted, that some of the used metal additives (such as Si, Zr,Ti, Al or Sn additives) will not yield completely fluorinatedcompounds—regardless of the applied (stoichiometric) amount of HF.

In some embodiments, a magnesium precursor and a metal additive are usedand essentially all of the magnesium and essentially the entire additiveis fluorinated. Thus, in such embodiments, the HF stoichiometry is fixedto exactly 2.0 in reference to the content of magnesium plus anadditional amount for the metal additive, wherein the additional amountof fluoride that can react with the additive is computed according tothe oxidation state of the additive and the amount of the additiveemployed in the reaction. For example, if 200 mmol ZnCl₂ (as a metaladditive) and 1 mol magnesium precursor are used, the amount of HF willbe 2.4 mole. The addition of the metal additive compound before addingthe HF solution allows a complete fluorination of the magnesiumprecursor and the metal additive compound.

In some embodiments, trace amounts of non-fluorinated compounds (1-2% ofthe magnesium employed in the reaction) may persist in the sol thusformed, as discussed above. An analogue description applies if two ormore different metal additives are used. In some embodiments, even withsuch a fixed HF stoichiometry only partially fluorinated additivecompounds are observed.

In some embodiments, a magnesium precursor and metal additives are usedand essentially all of the magnesium and some of the additive isfluorinated. Thus, in such embodiments, the HF stoichiometry is fixed to2.0 in reference to the content of magnesium derived from the magnesiumprecursor employed in the reaction, plus an additional amount of HF,which is computed multiplying the oxidation state of the additive, theamount of the additive and the molar percentage of additivefluorination. Thus, according to the above example of 1 mol magnesiumprecursor and 200 mmol zinc, the stoichiometry of HF can be selected inthe range from 2.0 mol HF (essentially no fluorination of the additivetakes place) to 2.4 mol HF (essentially all of the additive isfluorinated). If the aim of the reaction is only a partial fluorinationof the additive (e.g. of about 50% with reference to the above mentionedexample), 2.2 mol HF will be applied. Depending on the amount ofadditive present in the reaction, the amount of HF for a potentiallycomplete fluorination will be computed accordingly. For example, if 100mmol ZnCl₂ and 1 mol of magnesium precursor will be used, the amount ofHF can be selected in the range from 2.0 to 2.2 mole HF. The sameapplies for the oxidation state. A lower oxidation state amounts to alesser amount of HF necessary for a complete fluorination, wherein ahigher oxidation state amounts to a higher amount of HF. For example, ifa Li⁺-additive (200 mmol) is used (instead of the above mentionedZnCl₂), the range of HF will be between 2.0 and 2.2 mol HF, however, ifa Ti⁴⁺-additive (200 mmol) is used, the range of HF will be between 2.0and 2.8 mol HF. An analogue description applies for two or moreadditives.

In some embodiments, in particular if chloride is present, even ifstoichiometric amounts of HF are used, not all of the used HF will reactwith the magnesium precursor. In such a case a further metal additivemay be used, which will react with the unreacted HF, yielding somepartially fluorinated additive compounds while removing the remaining HFfrom the sol reaction. The inventors believe, without wishing to bebound by this theory, that unreacted HF may cause an inferiorwettability of the sol, which generally occurs after a few days, incomparison to other sols according to the invention comprising no orless unreacted HF Thus, the HF stoichiometry can be fixed to 2.0 inreference to the content of magnesium derived from the magnesiumprecursor (n_(Mg)) used in the reaction, plus an additional amount of HF(n_(adHF)) according to the following formula:n _(HF)=2*n _(Mg) +n _(adHF).

The HF stoichiometry for the additive can be computed according to theoxidation state (Ox) of the additive and the amount of the additive andthe selected degree of fluorination of said additive (A). The additionalamount of HF (n_(adHF)) can be computed according to the followingformula:n _(adHF)=(n _(Mg)*X_(additive))*Ox*A,

wherein n_(Mg) is the amount of magnesium precursor, X_(additve) is themolar percentage of said metal additive precursor (second magnesiumprecursor or non-magnesium precursor) in relation to said molar amountof said first magnesium precursor, Ox is the oxidation state of themetal of the additive and A is selected from 0≤A≤1.

In some embodiments, A is 0, yielding essentially unfluorinated metaladditive compounds and no additional HF is applied. In some embodiments,in cases of an incomplete reaction of the stoichiometric amount of HFand the magnesium precursor, A is 0 and no additional HF is applied,yielding unfluorinated and partially fluorinated metal additivecompounds, thus, removing or “neutralizing” the unreacted amount of HFIn some embodiments, A is 1, yielding essentially completely fluorinatedmetal additive compounds. In some embodiments, A is 0<A<1, yieldingpartially fluorinated metal additive compounds.

Depending on the metal of the additive used, the oxidation state of saidmetal, the amount of HF applied and the reaction conditions, theadditive can be completely fluorinated, leading to MF_(x) additives.Alternatively, parts of the additive can be completely fluorinated,leading to MF_(x)/MB_(x) mixtures of the additive as well as partiallyfluorinated additives MF_(m)B_(x-m) or mixtures thereof (e.g.MF_(m)B_(x-m)/MB_(x) or MF_(x)/MF_(m)B_(x-m)/MB_(x)), wherein x is equalto the oxidation state of the metal and m is equal to or smaller thanthe oxidation state and B is selected from the group comprising analcoholate (RO—), a carboxylate (RCOO—), a carbonate, analkoxycarbonate, a hydroxide or selected from the group of salts ofstrong, volatile acids, such as—by way of non-limiting example—chloride,bromide, iodide, nitrate or triflate, particularly alcoholate, nitrateor chloride. In some embodiments, B is selected from chloride,methanolate, ethanolate, propoxylate, buthylate, chloride or nitrate Thepartially fluorinated additives MF_(m)B_(x-m) can comprise of identicalfluorinated additives (e.g. only one residue is exchange by fluoride inthe metal additive) as well as partially fluorinated additives indifferent states of fluorination (e.g. in some additive particles oneresidue is exchange by fluoride in the metal additive and in otheradditive particles two residues are exchanged or other possiblecombinations). Similar arguments apply if a magnesium additive, asdiscussed above, is used and the amount of HF is stoichiometric (2 eqHF) or less (e.g. 1.85 to 1.95 eq HF).

In some embodiments, the metal additive compound is selected from thegroup of CaCl₂ MgCl₂, Ca(OEt)₂, LiCl, C₈H₂₀O₄Si, Ti(O^(i)Pr)₄,Al(O^(i)Pr)₃, AlCl₃, TiCl₄ and/or SiCl₄.

Clear sols with small MgF₂-particles between 3 and 10 nm are obtainedroutinely when the alkoxides are fully soluble in the non-aqueoussolvent, that is when clear alkoxide solutions are obtained. Startingfrom dispersed precursor systems often does not result in the formationof clear sols. Opaque or non-transparent sols do not give highperformance AR-layers. Dispersed precursor systems however have beenfound to clear quickly and render truly nanodisperse sol solutions whenadditive precursors as indicated in the previous paragraphs areemployed.

First Volume:

A large number of solvents offer themselves for practicing theinvention. In some embodiments, the non-aqueous solvent is an alcoholsuch as, by way of non-limiting example, methanol, ethanol orisopropanol. In some embodiments, the non-aqueous solvent is apolyalcohol such as, by way of non-limiting example, polyethyleneglycol. In some embodiments, the non-aqueous solvent is an ether suchas, by way of non-limiting example, diethyl ether, methyl tert-butylether or tetrahydrofurane. In some embodiments, the non-aqueous solventis an ester. In some embodiments, the non-aqueous solvent is a mixtureof any of the previously cited solvents.

For embodiments that aim at providing technical scale quantities, thesolvent needs to be available at low price. Since methanol is toxic,handling and operation of the sol formation itself and its downstreamapplications in surface coatings require additional safety measures. Forlarge scale applications, some embodiments of the process of theinvention employ ethanol or isopropanol as a solvent in particularethanol, as a solvent. For such applications, embodiments of the processof the invention are preferred in which ethanol or isopropanol can beused as a solvent. The method according to the present invention offersthe great advantage of providing sol solutions that are particularlystable in ethanol even at high concentrations. In some embodiments—usingethanol or isopropanol as a solvent—the solution has a magnesium contentin the range of about 0.2 mol/L to 0.8 mol/L, in particular of about 0.2mol/L to 0.6 mol/L. In some embodiments using ethanol or isopropanol, inparticular ethanol, as a solvent the solution has a magnesium content inthe range of about 0.2 mol/L to 0.4 mol/L. In some embodiments, themagnesium content is 0.2 mol/L, 0.3 mol/L, 0.4 mol/L, 0.5 mol/L, 0.6mol/L, 0.7 mol/L or 0.8 mol/L.

In some embodiments, the magnesium precursor is magnesium methylate. Inone embodiment, the non-aqueous solvent is ethanol or isopropanol. Inone embodiment, the non-aqueous solvent is ethanol or isopropanol andthe precursor is magnesium ethylate.

An important distinction of embodiments making use of additives is thefact that the additive precursor may be added during the fluorinationstep, i.e. along with the magnesium alkoxide precursor. The inorganicacids produced during the fluorination step result in a faster clearingof the sol solution. In some embodiments making use of additives theadditive precursor may be added after the fluorination step of the firstmagnesium precursor. In some embodiments, the additive precursor ispresent in an amount of 0.2 mole (1:5) to 0.01 mole (1:100) per molemagnesium alkoxide.

Among the alcohol-magnesium alcoholate systems, magnesium methylate issoluble in methanol only, and the ethylate does not lead to clearsolutions suitable to generation of fluoride sols in any alcohol,according to the observation of the present inventors. The method of thepresent invention therefore provides an alternative route to the acetateprocess commonly employed to obtain MgF₂ sol solutions for coatingpurposes. Even this acetate route uses methanol. The present inventionin contrast provides a process that facilitates the use of ethanol andmagnesium ethoxide, as one example, leading to commercially andtechnically important improvements in the process of obtaining solsolutions for coating.

In some embodiments, the concentration of the magnesium alkoxide in thefirst volume is 0.1 mol/l, 0.2 mol/l, 0.3 mol/l, 0.4 mol/l, 0.5 mol/l,0.6 mol/l, 0.7 mol/l, 0.8 mol/l, 0.9 mol/l, 1.0 mol/l, 1.2 mol/l, 1.3mol/l, 1.4 mol/l or 1.5 mol/l.

Second Volume:

In some embodiments, the HF is added in liquid solution. In someembodiments, the HF solution has a concentration of 0.5 mol/l, 1 mol/l,2 mol/l, 3 mol/l, 4 mol/l, 5 mol/l, 6 mol/l, 8 mol/l, 10 mol/l, 15mol/l, 20 mol/l, 25 mol/l or 30 mol/l.

In some embodiments, the required HF is added in solution during 5 minto 60 min under intense stirring while keeping the temperature at orbelow 35° C.

In some embodiments, the HF is added as a gas.

According to a second aspect of the invention, a magnesium fluoride solsolution obtainable by a method according to the first or second aspectof the invention is provided. As an alternative of this second aspect ofthe invention, a magnesium fluoride sol solution comprising MgF₂particles in a non-aqueous solvent is provided. The magnesium fluoridesol solution of the invention is characterized by a particle diametersize of smaller than (<) 100 nm, <75 nm, <50 nm, <40 nm, <30 nm, <20 nmor <10 nm. In some embodiments, the magnesium fluoride sol solution isconstituted by nanoscopic scale MgF₂ particles with particle sizesranging from 3 to 60 nm. In some embodiments, the particle size rangesfrom 3 to 20 nm, in particular 3 to 15. In some embodiments, theparticle size ranges from 3 to 10 nm.

The resulting additive fluoride particles may not be double salts ofmagnesium and additive, but rather distinct species, present in the solas a mixture of single components (MgF₂ and the MF_(x) additive or MgF₂and the MB_(x) or additive, respectively). In some embodiments, theadditive fluoride particles have a diameter size of smaller than (<) 50nm, <40 nm, <30 nm, <20 nm, <10 nm, <5 nm, <4 nm or <3 nm. In someembodiments, the additive fluoride particles have a particle diametersize of smaller than (<) 20 nm, 15 nm, <10 nm, <7 nm, <5 nm, <4 nm, or<3 nm. The same applies if more than two metal components are used.

Depending on the reaction conditions resulting additive fluorideparticles may be double salts of magnesium and additive present in thesol as a mixture of double salt components (MgF₂/MF_(x) additive orMgF₂/MB_(x) or MF_(m)B_(x-m) additives, respectively, as discussedabove). In some embodiments, the double salt component particles have adiameter size of smaller than (<) 50 nm, <40 nm, <30 nm, <20 nm, <10 nm,<5 nm, <4 nm or <3 nm. In some embodiments, the double salt componentparticles have a particle diameter size of smaller than (<) 20 nm, 15nm, <10 nm, <7 nm, <5 nm, <4 nm, or <3 nm. The same applies, if morethan two metal components are used, yielding multiple salt components.

In some embodiments, the sol reaction results in a mixture of the singlecomponent particles and the double (multiple) salt particles with theparameters as discussed above.

The particle size can be adjusted by varying the parameters of theprocess according to the first or second aspect of the invention.Formation of smaller particles is favored at temperatures between 20° C.and 30° C., slow addition of HF (over 30 to 60 minutes) and very highspeeds of stirring (300 rpm). In some embodiments a stirring of about600 to 1000 rpm is applied. Lower temperatures or slower stirring willgenerally favor formation of larger particles.

In some embodiments, said MgF₂ particles are smaller than 10 nm in anon-aqueous solvent, and the sol comprises additive particles, which aresmaller than 50 nm in diameter.

The solvent may be selected as discussed above in respect to the methodof the invention.

In some embodiments, the non-aqueous solvent is methanol, ethanol orisopropanol. In some embodiments, the non-aqueous solvent is methanol.In some embodiments, the non-aqueous solvent is ethanol or isopropanol.In some embodiments, the non-aqueous solvent is ethanol.

The sol solution of the invention is characterized by the presence ofmolecular species formed by the reaction of HF with the reactivemagnesium precursor. Thus, to the extent that Mg—O—Mg complexes werepresent prior to addition of carbon dioxide, alkoxycarbonic acid is acharacteristic component of the sol solution.

To the extent that a second magnesium fluoride precursor was present inthe first volume, the corresponding acid, such as by way of non-limitingexample, HCl, HBr or HI, will be present in the sol solution. If anon-magnesium precursor was present, a broader range of anions can beemployed, leading to different reaction products (corresponding acids)such as acetates or lactates. These in turn will go on to react with thesolvent, if the solvent is an alcohol, to form the corresponding esters.Acetates are esterified to a great extent (almost 100%), whereaslactates esterify only to about 30%.

In some embodiments, the magnesium fluoride sol solution of theinvention is characterized by a solution comprising an amount of MgF₂particles and, additionally, an amount of additive particles, derivedfrom a magnesium additive and/or a metal additive (as described above),characterized by a general formula MF_(m)B_(x-m), wherein M^(n+) isselected from the group of Li⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sn²⁺, Zn²⁺,Al³⁺, Si⁴⁺, Ti⁴⁺, Zr⁴⁺, Sn⁴⁺, Sb³⁺ or Sb⁵⁺, B is an anionic ligand to M,x is equal to the oxidation state of the metal M and m is equal to orsmaller than the oxidation state of the metal M. In some embodiments, Bis selected from the group of strong, volatile acids, such as—by way ofnon-limiting example—a chloride, bromide, iodide, nitrate or triflate.In some embodiments, B is selected from chloride or nitrate, inparticular chloride.

In some embodiments, the sol solution comprises essentially only fullyfluorinated additive particles, wherein essentially all ligand B hasbeen exchanged for fluoride, in other words, the additive component ofthe sol solution is characterized by a formula MF_(x), with x equal tothe oxidation state of the metal M.

In some embodiments, the sol solution comprises essentially onlypartially fluorinated additive particles, wherein some but not all ofligand B has been exchanged for fluoride, in other words, the additivecomponent of the sol solution is characterized by a formulaMF_(m)B_(x-m), with m being in the range of 0<m<x. In some suchembodiments, m is 0.1x, 0.2x, 0.3x, 0.4x, 0.5x, 0.6x, 0.7x, 0.8x, or0.9x. In some embodiments, m is in the range of 0<m<x, wherein x is theoxidation state of the metal M and m and x are natural numbers.

In some embodiments, the sol solution may comprise also someunfluorinated magnesium particles, as discussed above. An example ofsuch an unfluorinated additive particle is MgCl₂, which is due to anincomplete reaction of HF with the MgCl₂ precursor (optionally a“neutralisation” of the remaining HF by adding a metal additive, asdescribed above may be applied). The amount of such unfluorinatedmagnesium precursors is around 1 to 2% in relation to the amount of usedmagnesium precursor. Additionally the amount of unfluorinated magnesiumprecursors may be elevated by using less than a stoichiometric amount ofHF.

In some embodiments, the sol solution comprises essentially onlynon-fluorinated additive particles, wherein essentially none of ligand Bhas been exchanged for fluoride, in other words, the additive componentof the sol solution is characterized by a formula MB_(x).

In some embodiments, the sol solution comprises fully fluorinatedadditive particles and/or partially fluorinated additive particlesand/or non-fluorinated particles in a combination. Additionallypartially fluorinated first magnesium precursor particles andnon-fluorinated first magnesium precursor particles may be present.

In some embodiments, the sol solution comprises MgF₂ particles andMB_(x) metal additive particles, wherein M is selected from the group ofLi⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sn²⁺, Zn²⁺, Al³⁺, Si⁴⁺, Ti⁴⁺, Zr⁴⁺, Sn⁴⁺,Sb³⁺ or Sb⁵⁺, and B is an anionic ligand, and x is equal to theoxidation state of the metal M. B can be selected from the group ofsalts of strong, volatile acids, such as—by way of non-limitingexample—chloride, bromide, iodide, nitrate or triflate, in particularchloride or nitrate. In some embodiments, M^(n+) is selected from thegroup comprising Li⁺, Mg²⁺, Ca²⁺, Al³⁺, Si⁴⁺ or Ti⁴⁺.

In some embodiments, the sol solution comprises MgF₂ particles andMF_(x) metal additive particles, wherein M is selected from the group ofLi⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sn²⁺, Zn²⁺, Al³⁺, Si⁴⁺, Ti⁴⁺, Zr⁴⁺, Sn⁴⁺,Sb³⁺ or Sb⁵⁺, and B is an anionic ligand, and x is equal to theoxidation state of the metal M. In some embodiments, M^(n+) is selectedfrom the group comprising Li⁺, Mg²⁺, Ca²⁺, Al³⁺, Si⁴⁺ or Ti⁴⁺.

In some embodiments, the magnesium fluoride sol solution of theinvention comprises MgF₂ particles and an amount of additive particlesof the formula MF_(m)B_(x-m), wherein M^(n+) is selected from the groupcomprising Li⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sn²⁺, Zn²⁺, Al³⁺, Si⁴⁺, Ti⁴⁺,Zr⁴⁺, Sn⁴⁺, Sb³⁺ or Sb⁵⁺, wherein x is equal to the oxidation state ofM, m is equal to or smaller than x, B is selected from the group ofsalts of strong, volatile acids, such as—by way of non-limitingexample—chloride, bromide, iodide, nitrate or triflate, in particularchloride or nitrate. In some embodiments, M^(n+) is selected from thegroup comprising Li⁺, Mg²⁺, Ca²⁺, Al³⁺, Si⁴⁺ or Ti⁴⁺.

The sol solution comprising fully fluorinated, partially fluorinated ornon-fluorinated additive particles MF_(m)B_(x-m) can be provided by amethod as described above, using no additional amount of HF, anadditional amount of HF (n_(adHF)) or using a stoichiometric amount ofHF with respect to the applied magnesium precursor (see the discussionabove concerning essentially quantitative conversion of the additive).The sol solution can comprise, depending on the conditions applied, onlyMB_(x), MF_(x) or MF_(m)B_(x-m) additives as well as mixtures thereof.(e.g. MF_(m)B_(x-m)/MB_(x) or MF_(x)/MF_(m)B_(x-m)/MB_(x)).

Where a particular application requires, the properties of theMgF₂-layers can be adjusted regarding their mechanical and/or opticalproperties by introducing further M^(n+)-additives (by way ofnon-limiting example, M^(n+)=Ca²⁺, Sr²⁺, Ba²⁺, Zn²⁺, Al³⁺, and Si⁴⁺)directly to the Mg²⁺-precursor solution. Surprisingly, if theseprecursors are added to the Mg²⁺-precursor solution before adding therequested stoichiometric amount HF, the sols clear more quickly andresulting MgF₂-layers show significantly improved and distinctlydifferent properties as compared to the post addition of e.g. CaF₂-,SrF₂-, AlF₃-, or SiO₂-particles to the synthesized pure MgF₂-sol. Insome embodiments, the magnesium fluoride sol solution of the inventioncomprises an amount of additive particles selected from the groupcomprising CaF₂, SrF₂, BaF₂, AlF₃, SiF₄, ZrF₄, TiF₄, and/or ZnF₂. Insome embodiments, the sol solution comprises MgF₂ particles and anamount of additive particles selected from the group comprising LiF,CaF₂, SrF₂, BaF₂, SnF₂, AlF₃, SbF₃, SbF₅, ZrF₄, and/or ZnF₂. In someembodiments, the amount of said additive particles, in relation to theamount of MgF₂ particles, is 1:5 to 1:500, as measured in molarequivalent of additive to magnesium.

The inventor believes that the prior addition of the metal additivecompound—before adding the requested stoichiometric amount HF—positivelyinfluences the seed crystal and particle generation, leading to betterdispersion of the non-magnesium additives.

Where a particular application requires, the properties of theMgF₂-layers can be adjusted regarding their mechanical and/or opticalproperties by introducing further metal additives (see precedingsection), by the method described above. If these metal additivecompounds are added to the magnesium precursor after adding therequested stoichiometric amount of HF and no additional amount of HF oran additional amount of HF, which is less than the stoichiometric amountwith respect to the amount of metal additive (see discussion concerninga partially fluorinated additive) the resulting MgF₂-layers showsignificantly improved and distinctly different properties as comparedto a solution where metal additives are added before HF is added to thesol reaction. Thus, even better results can be obtained, if the metaladditives are added after adding the requested stoichiometric amount ofHF.

The inventors believe, without wishing to be bound by this theory, thatthis is due to a higher amount of a free electrolyte concentration andthe reaction of eventually unreacted HF (as discussed) with the metaladditive, which leads to a stabilization of the particles in the sol.

In some embodiments, the sol solution is obtained according to a methoddescribed according to the first or second aspect of the invention,wherein the metal additive is added after the fluorination with HF(computed according to the amount of magnesium precursor). In someembodiments, the magnesium fluoride sol solution of the inventioncomprises an amount of additive particles selected from the groupcomprising CaCl₂ MgCl₂, Ca(OEt)₂, LiCl, C₈H₂₀O₄Si, Ti(O^(i)Pr)₄,Al(O^(i)Pr)₃, AlCl₃, TiCl₄ and/or SiCl₄, or partially fluorinatedparticles thereof.

In some embodiments, said amount of metal additive particles (derivedfrom non-magnesium additives or second magnesium additives, as describedabove) in the sol solution is 1:5 to 1:100, as measured in molarequivalent of additive to magnesium precursor. Thus, the metal additivecompound is present in an amount of 0.2 mole (1:5) to 0.01 mole (1:100)per mole to magnesium precursor. In some embodiments, the metal additivecompound is present in an amount of 0.2 mole (1:5) to 0.04 mole (1:25)per mole to magnesium precursor. In some embodiments, the metal additivecompound is present in an amount of 0.2 mole (1:5) to 0.08 mole (1:12.5)per mole to magnesium precursor. In some embodiments, the metal additivecompound is present in an amount of 0.1 mole (1:10) to 0.04 mole (1:25)per mole to magnesium precursor. In some embodiments, the metal additivecompound is present in an amount of 0.04 mole (1:25) to 0.01 mole(1:100) per mole to magnesium precursor. This applies also foressentially completely fluorinated, partially fluorinated or essentiallyunfluorinated metal additives.

It is understood that the sol may comprise in particular the specifiedcompounds and specified amounts of these compounds as described in theembodiments concerning the method for obtaining a magnesium sol.

In some embodiments, said additive particles are smaller than 50 nm indiameter.

In some embodiments, said MgF₂ particles are smaller than 10 nm in anon-aqueous solvent, and said sol comprises additive particles, whichare smaller than 50 nm in diameter.

In some embodiments, said sol comprises double salts of MgF₂ particlesand additive particles, which are smaller than 50 nm in diameter.

An important parameter in handling nanoparticle sol solutions forindustrial applications is the concentration of the solution, withhigher concentrations allowing more options to the user. Highconcentrations additionally lead to lower costs in transporting andstoring the solution prior to its use. The sol solutions of theinvention show distinctly greater stability at high concentrations whencompared to sol solutions of the art.

In some embodiments, the magnesium fluoride sol solution of theinvention has a magnesium content of larger than (>) 0.2 mol/l, 0.4mol/l, 0.5 mol/l, 0.6 mol/l, 0.7 mol/l, 0.8 mol/l, 0.9 mol/l, 1.0 mol/l.

The concentration of the MgF₂-sol can range from 0.05 to 1 mol/l. Insome embodiments, the concentration is between 0.15 and 0.5 mol/l. Insome embodiments, the magnesium fluoride sol solution of the inventionhas a magnesium content of 0.6 mol/L. In some embodiments, the magnesiumfluoride sol solution using ethanol or isopropanol, particularlyethanol, of the invention has a magnesium content in the range of about0.2 mol/L to 0.8 mol/L, in particular of about 0.2 mol/L to 0.6 mol/L.In some embodiments, the magnesium fluoride sol solution using ethanolor isopropanol of the invention has a magnesium content in the range ofabout 0.2 mol/L to 0.8 mol/L, in particular 0.2 mol/L to 0.4 mol/L.

In some embodiments, the dynamic viscosity of the sol solution is in therange of 1.0 to 8.0 mPa s, in particular in the range of about 1.3 to3.5 mPa s.

To some extent, the maximum concentration of the sol solution has beenfound to depend on the educt components. One particularly stable sol isobtained by reacting magnesium methylate or ethylate in methanol,ethanol or isopropanol, wherein solutions up to 1 mol/l can be obtainedand stored without losing the distinct clarity of the solution over manymonths.

In some embodiments, the magnesium fluoride sol solution of theinvention is stable at room temperature (20° C.) for more than 10 weeks.

Such stability is of advantage not only because gelling of the solsolution (which is generally irreversible) leads to material loss, butalso because the recipients used to store the gelled solution need to becleaned, which can be a laborious and expensive task.

According to a third aspect of the invention, a method for coating asurface is provided, comprising the steps of

-   -   a. providing a magnesium fluoride sol solution;    -   b. contacting the surface with the magnesium fluoride sol        solution;    -   c. drying said surface; and    -   d. exposing said surface in a first thermal step to a first        temperature ranging from 15° C. to 500° C., in particular 200 to        500° C.

In some embodiments, a method for coating a surface is provided,comprising the steps of

-   -   a. providing a magnesium fluoride sol solution;    -   b. contacting the surface with the magnesium fluoride sol        solution;    -   c. drying said surface; and    -   d. exposing said surface in a first thermal step to a first        temperature ranging from 15° C. to 100° C.

The magnesium fluoride sol solution has been obtained, or has thequalities of a sol solution that can be obtained, by a method accordingto the first or second aspect of the invention. Alternatively, a solsolution having the qualities described above is employed.

A number of methods of applying the sol solution to the surface offerthemselves. In some embodiments, the coating is applied by spin coatingor by dip-coating. In some embodiments, the coating is applied byspraying. The person skilled in the art will recognize the most suitableprocess of application depending on the shape of the substrate surfaceto be coated.

In some embodiments, the dynamic viscosity of the sol solution is in therange of 0.8 until 5.0 mPa s, in particular in the range pf 1.3 to 3.5mPa s.

The drying and first thermal step allow for the removal of solvent andresidual non-fluoridic components of the sol solution, mainly the acidformed by reacting of the precursor compound(s) with HF, or anysubsequent reaction products derived thereof, such as esters. Dependingon the compounds used in making the sol solution, the requirements fortime and temperature of the process will vary from room temperature (20°C.) to temperatures higher than 100° C., and from a few minutes toseveral hours. In one embodiment, the first thermal step proceedsbetween 250 and 500° C. for 5 to 30 min. In some embodiments, the firstthermal step proceeds at between 300 and 400° C. for 5 to 30 min. In allcases the thermal treatment can be performed at ambient pressure.

In some embodiments, the drying takes place during the time intervalbetween applying the coating on the surface and exposing the surface toa thermal step (e.g. the time between removal of the surface from thesol solution after dipping the surface in the sol solution and bringingthe surface to an application for exposing the surface to thermal step).The drying allows for the removal of a large part of solvent andresidual components of the sol solution, however, not all thesecomponents can be removed completely. Some will remain under theseconditions in pores of the coating. The aim of the drying is to providea smear-resistant coating.

In some embodiments, the drying step occurs for 10 min at roomtemperature.

In some embodiments, a thermal step is applied, wherein said surface isexposed to a temperature ranging from 100° C. to 500° C. In someembodiments a temperature in the range of about 250° C. to 500° C. isused. In some embodiments a temperature in the range of about 400° C. to500° C. is used. This thermal step may be employed to sinter thecoating, leading to improved mechanical stability.

In some embodiments, the thermal step will be applied to the surface insuch a way that the surface is directly exposed to the necessarytemperature, e.g. the surface is exposed to 450 C. Alternatively thesurface can be exposed to a slowly increasing temperature interval untilthe necessary temperature (e.g. 450° C.) is reached. Alternativelyhigher temperatures may be applied if necessary. In some embodiments,the thermal step has a duration of 5 to 30 min. In all cases thesethermal treatment can be performed at ambient pressure. In someembodiments, the thermal step (at a temperature of 450° C.) has aduration of 15 min. In some embodiments, the coating is allowed to cooldown slowly over longer period of time (preferential between 100 and 150min). The slow cooling of the heated coating will result in bettercharacteristics concerning the mechanical properties of the coating. Insome embodiments, the surface is exposed after the drying and prior tothe thermal step to an additional drying temperature, wherein saidsurface is exposed to a drying temperature ranging from 15° C. to 100°C. for a certain amount of time.

The drying and first thermal step allow for the removal of solvent andresidual non-fluoridic components of the sol solution, mainly the acidformed by reacting of the precursor compound(s) with HF, or anysubsequent reaction products derived thereof, such as esters. Dependingon the compounds used in making the sol solution, the requirements fortime and temperature of the process will vary from room temperature totemperatures higher than 100° C., and from a few minutes to severalhours. The drying and the additional drying temperature allow for theremoval of a large part of solvent and residual components of the solsolution, however, not all these components can be removed completely.Some will remain under these conditions in pores of the coating. The aimof the drying and the additional drying temperature is to provide asmear-resistant coating.

In some embodiments, 80° C. is used for the additional dryingtemperature which occurs for 10 min.

In one embodiment, the first thermal step proceeds between 250 and 500°C. for 5 to 30 min. In some embodiments, the first thermal step proceedsat between 300 and 400° C. for 5 to 30 min. In all cases the thermaltreatment can be performed at ambient pressure.

In some embodiments, after the first thermal step, a second thermal stepis applied wherein said surface is exposed to a second temperatureranging from 250° C. to 500° C. This second thermal step may be employedto sinter the coating, leading to improved mechanical stability.

In some embodiments, one thermal post-treatment is applied afterdepositing the MgF₂-layer.

In some embodiments, a two-step thermal process is applied, wherein thesubstrate is treated in a first thermal step between room temperatureand 100° C., and in a second thermal step at between 200 and 500° C. Insome embodiments, the first thermal step has a duration of 5 to 60 min.In some embodiments, the second thermal step has a duration of 5 to 30min. In one embodiment, a two-step thermal process has a first thermalstep between 70° C. and 90° C. for 5 to 60 min, and a second thermalstep between 250° C. and 500° C. for 5 to 30 min. In some embodiments,the second thermal step proceeds at between 300° C. and 400° C. for 5 to30 min. In all cases the thermal treatment can be performed at ambientpressure.

In some embodiments, after the additional drying temperature, thethermal step is applied (two-step thermal process), wherein said surfaceis exposed to a temperature ranging from 100° C. to 500° C. In someembodiments the temperature of the thermal step is in the range of about250° C. to 500° C. In some embodiments, a temperature in the range ofabout 400° C. to 500° C. is used for the thermal step. This thermal stepmay be employed to sinter the coating, leading to improved mechanicalstability.

In some embodiments, the thermal step will be applied separately afterthe additional drying temperature. It is thus possible to allow for acooling of the coating after the additional drying temperature. In someembodiments, the thermal step will be applied directly after theapplication of the additional drying temperature, thus, no cooling ofthe coating is allowed until after the thermal step is finished. In someembodiments, the thermal step will be applied directly after theadditional drying temperature, wherein the coating is heated slowlyuntil the necessary end temperature is reached (e.g. 450° C.).

In some embodiments, the additional drying temperature exposure has aduration of 5 to 60 min. In some embodiments, the thermal step has aduration of 5 to 30 min.

In some embodiments, a two-step thermal process is applied, wherein thesubstrate is treated in a first thermal step between room temperatureand 100° C., and in a second thermal step at between 200° C. and 500° C.In some embodiments, the first thermal step has a duration of 5 to 60min. In some embodiments, the second thermal step has a duration of 5 to30 min. In one embodiment, a two-step thermal process has a firstthermal step between 70 and 90° C. for 5 to 60 min, and a second thermalstep between 250° C. and 500° C. for 5 to 30 min. In some embodiments,the second thermal step proceeds at between 300° C. and 400° C. for 5 to30 min. In all cases the thermal treatment can be performed at ambientpressure. In some embodiments, the additional drying temperatureexposure (temperature 80° C.) has a duration of 10 min, wherein thethermal step (at a temperature of 450° C.) has a duration of 15 min. Insome embodiments, the coating is allowed to cool down slowly over longerperiod of time (preferential between 100 and 150 min). The slow coolingof the heated coating will result in better characteristics concerningthe mechanical properties of the coating.

In some embodiments, the substrate is heated slowly (2K to 5K per min),held isothermically for 10 to 45 min and is cooled subsequently at asimilar rate (2K to 5K per min). This treatment has shown particularlysatisfactory results as regards mechanic stability.

Any of the sol solutions provided herein will result in porous andmechanically robust layers that can be obtained in just a singlecoatings step.

According to a fourth aspect of the invention, a surface coatingcomprising magnesium fluoride nanoparticles is provided. This coatingmay be characterized by its constituent particles' size, its porosity,optical qualities and/or scratch resistance.

In some embodiments, the surface coating of the invention ischaracterized by a refractivity index between n₅₀₀=1.21 and n₅₀₀=1.32.In some embodiments, the surface coating of the invention ischaracterized by a porosity of 30 to 50%. In some embodiments, thesurface coating of the invention is characterized by a porosity of 40 to55%. In some embodiments, the surface coating of the invention ischaracterized by a refractivity index between n₅₀₀=1.21 and n₅₀₀=1.32and a porosity of 30 to 50%.

In some embodiments, the surface coating comprises an amount of additiveparticles selected from the group comprising CaF₂, SrF₂, BaF₂, AlF₃,SiF₄, ZrF₄, TiF₄, and/or ZnF₂, said amount being 1:5 to 1:500 asmeasured in molar equivalent of additive to magnesium. In someembodiments, the additive particles are comprised in a ratio of 1:100,1:10, 1:20, 1:30, 1:40 or 1:50 as measured in molar equivalent ofadditive to magnesium.

In some embodiments, the surface coating of the invention ischaracterized by a porosity of 30 to 50%. In some embodiments, thesurface coating of the invention is characterized by a porosity of 40 to55%.

In some embodiments, the surface coating of the invention ischaracterized by a refractivity index between n₅₀₀=1.21 and n₅₀₀=1.32.

In some embodiments, the surface coating of the invention ischaracterized by a refractivity index between n₅₀₀=1.21 and n₅₀₀=1.32, aporosity of 30 to 50% and an amount of additive particles selected fromthe group comprising LiF, CaF₂, SrF₂, BaF₂, SnF₂, AlF₃, SbF₃, SbF₅,ZrF₄, and/or ZnF₂. said amount being 1:5 to 1:500 as measured in molarequivalent of additive to magnesium. In some embodiments, the additiveparticles are comprised in a ratio of 1:100, 1:10, 1:20, 1:30, 1:40 or1:50 as measured in molar equivalent of additive to magnesium.

According to yet another alternative of this fourth aspect of theinvention, a surface coating comprising magnesium fluoride nanoparticlesis provided, characterized by a refractive index of between n₅₀₀=1.18and n₅₀₀=1.35, a porosity of 25% to 40% and an amount of additiveMF_(m)B_(x-m) particles (derived from non-magnesium additives or secondmagnesium additives, as described above), wherein M is selected from thegroup of Li⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sn²⁺, Zn²⁺, Al³⁺, Si⁴⁺, Ti⁴⁺,Zr⁴⁺, Sn⁴⁺, Sb³⁺ or Sb⁵⁺, B can be a chloride or an oxide, x is equal tothe oxidation state of the metal M and m is equal to or smaller than theoxidation state of the metal M, said amount being 1:5 to 1:100, asmeasured in molar equivalent of additive to magnesium.

In some embodiments, the surface coating comprises an amount additiveparticles (derived from non-magnesium additives or second magnesiumadditives, as described above) M^(n+)F_(m)B_(x-m) with m equal to theoxidation state n of the metal M (as defined above) or with m equal to 0or with m selected from the range of 0<m<n or mixtures thereof, saidamount being 1:5 to 1:100, as measured in molar equivalent of additiveto magnesium derived from the first magnesium precursor.

In general, if magnesium (or another metal) chloride or fluorideparticles are present in the sol, they remain as the respectivechlorides or fluorides in the surface coating, however, the otherapplied ligands will yield the respective magnesium (or other appliedmetal) oxides.

In some embodiments, the coating comprises metal additives in the sameamount used in the previously discussed method for obtaining said solsolution.

In some embodiments, the surface coating comprises amount of an amountof additive MF_(m)B_(x-m) particles (derived from non-magnesiumadditives or second magnesium additives, as described above), wherein Mis selected from the group of Li⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sn²⁺, Zn²⁺,Al³⁺, Si⁴⁺, Ti⁴⁺, Zr⁴⁺, Sn⁴⁺, Sb³⁺ or Sb⁵⁺, B can be a chloride or anoxide, x is equal to the oxidation state of the metal M and m is equalto or smaller than the oxidation state of the metal M, said amount being1:5 to 1:100, as measured in molar equivalent of additive to magnesium.In some embodiments, the amount of additive ranges from 1:10 (10%) to1:25 (4%) as measured in molar equivalent of additive to magnesium. Inone embodiment, the amount of additive ranges from 1:12.5 (8%) to 1:25(4%) as measured in molar equivalent of additive to magnesium. In someembodiments, the surface coating comprises an amount additive particles(derived from non-magnesium additives or second magnesium additives, asdescribed above) M^(n+)F_(m)B_(x-m) with m equal to the oxidation staten of the metal M (as defined above) or with m equal to 0 or with mselected from the range of 0<m<n or mixtures thereof. Thus, the surfacecoating may comprise completely fluorinated additive particles,partially fluorinated additive particles or unfluorinated additiveparticles.

In some embodiments, the surface coating comprises amount of an amountof additive MF_(m)B_(x-m) particles (derived from non-magnesiumadditives or second magnesium additives, as described above), wherein Mis selected from the group of Li⁺, Mg²⁺, Ca²⁺, Al³⁺, Si⁴⁺ or Ti⁴⁺, B canbe a chloride or an oxide, x is equal to the oxidation state of themetal M and m is equal to or smaller than the oxidation state of themetal M, said amount being 1:5 to 1:100, as measured in molar equivalentof additive to magnesium. In some embodiments, the amount of additiveranges from 1:10 (10%) to 1:25 (4%) as measured in molar equivalent ofadditive to magnesium. In one embodiment, the amount of additive rangesfrom 1:12.5 (8%) to 1:25 (4%) as measured in molar equivalent ofadditive to magnesium. In some embodiments, the surface coatingcomprises an amount additive particles (derived from non-magnesiumadditives or second magnesium additives, as described above)M^(n+)F_(m)B_(x-m) with m equal to the oxidation state n of the metal M(as defined above) or with m equal to 0 or with m selected from therange of 0<m<n or mixtures thereof. Thus, the surface coating maycomprise completely fluorinated additive particles, partiallyfluorinated additive particles or unfluorinated additive particles.

In some embodiments, the surface coating comprises an amount of additiveparticles selected from LiX, wherein X is a halogen, in particularchloride, said amount being 1:5 to 1:100 (20% to 1%), as measured inmolar equivalent of additive to magnesium. In one embodiment, the amountof LiX additive ranges from 1:10 (10%) to 1:100 (1%) as measured inmolar equivalent of additive to magnesium. In one embodiment, the amountof LiX additive ranges from 1:25 (4%) to 1:100 (1%) as measured in molarequivalent of additive to magnesium. In one embodiment, the amount ofLiX additive is 1:100 (1%) as measured in molar equivalent of additiveto magnesium.

In some embodiments, the surface coating comprises an amount of additiveparticles selected from the group comprising LiF, CaF₂, SrF₂, BaF₂,SnF₂, AlF₃, SbF₃, SbF₅, ZrF₄, and/or ZnF₂, said amount being 1:5 to1:1000, as measured in molar equivalent of additive to magnesium. In oneembodiment, the amount of additive ranges from 1:20 (5%) to 1:100 (1%)as measured in molar equivalent of additive to magnesium.

In the deposited MgF₂-layer, the additive compounds incorporated intothe AR-layers are X-ray amorphous. The high degree of structuraldistortion caused by the synthesis of the invention provides excitingoptical and mechanic properties.

According to a fifth aspect of the invention, a metal, a glass, apolymer, in particular an organic polymer, or a thermoplast surfacehaving a surface coating according to any of the above aspects of theinvention is provided.

In some embodiments, the substrate that is to be coated is a planar or atubular glass surface. In some embodiments, the substrate that is to becoated is a metal surface. In some embodiments, the substrate that is tobe coated is an organic polymer surface. In some embodiments, thesurface is a thermoplast surface.

In some embodiments, the surface is the surface of an optical system, byway of non-limiting example of a camera, an opthalmological lense, abinocular, a microscope or a similar optical device.

In some embodiments, a MgF₂-layer obtained by a process or a solaccording to the invention exhibits a refractive indexe ranging from1.21 up to 1.30 depending on the composition of the MgF₂-sol and thetemperature of the post-treatment procedure. The resulting compositionof the MgF₂-sol depends e.g. on the amount of water carried into orproduced by the sol reaction, the nature of eventual additives (Mg, Sr,Ba, Al, Si, Zr, Zn etc., in particular Li⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺,Sn²⁺, Zn²⁺, Al³⁺, Si⁴⁺, Ti⁴⁺, Zr⁴⁺, Sn⁴⁺, Sb³⁺ or Sb⁵⁺) and theirconcentration, and the temperature of the post-treatment. Generally,some additives increase the sintering ability of the MgF₂, thus,resulting in an increased mechanical robustness of the AR-layer. WhereasAl- and Li-additives do not impact the exciting optical properties,additives like Sn, Ti or Zr decrease slightly the optical transmittancedue to their higher refractive indices. The inventor has found that inparticular additive components (which can be a completely, a partiallyor a non-fluorinated compound as described above) with a low meltingpoint increase the sintering ability and allow an increased mechanicalrobustness while comprising a higher porosity, thus, the influence onthe optical properties is small and only a slight decrease, if at all,of the optical transmittance can be observed. Hence, the optimum contentof additives has to be used in order to obtain high opticaltransmittance and high mechanical stability of the AR-MgF₂-layers asdescribed above.

In some embodiments, for which the surface that is to be coated is anorganic polymer surface, methanol based sol solutions are preferred. Insome embodiments, the surface to be coated is an organic polymer surfaceand the sol solution comprises between 0% and 20% of additive particlesselected from the group MF_(m)B_(x-m) as described previously. In someembodiments, the surface to be coated is an organic polymer surface andthe sol solution comprises between 0% and 15% of additive particlesselected from the group MF_(m)B_(x-m) as described previously. In someembodiments, the surface to be coated is an organic polymer surface andthe sol solution comprises between 2% and 8% of additive particlesselected from the group MF_(m)B_(x-m) as described previously. In someembodiments, the surface to be coated is an organic polymer surface andthe sol solution comprises between 0% and 2% of additive particlesselected from the group MF_(m)B_(x-m) as described previously.

In some embodiments, the surface is an organic polymer surfacepretreated by a layer of polysilazane or by an inorganic networkobtained through controlled hydrolysis and condensation of organicallymodified Si alkoxides (ORMOCER, Fraunhofer-Institut für SilicatforschungISC, Würzburg, Germany). In some embodiments, the surface is an organicpolymer surface pretreated by corona poling.

The sol solutions provided by the present invention are particularlywell suited for coating methods that benefit from or require lowpost-coating treatment temperatures, as incompletely reacted magnesiumalkoxide precursors are decomposed at relatively low temperatures incomparison to the carboxylate precursors commonly employed.

In some embodiments, a MgF₂-layer obtained by a process according to theinvention exhibit a mechanic stability and scratch resistance superiorto that of SiO₂ porous AR layers known in the art.

The AR layers obtained according the present invention are characterizedby an easy production starting from commercially available precursors.The sols as prepared by the method of the invention can directly be usedfor coating any substrate.

No other components like metal complexes, organic binders, co-polymersetc. have to be added to the sols in order to create fine-tuned porosityinside the layers. For some polymers, the presence of a mediator layerbelow or a cover layer above the MgF₂-layer is not necessary, but may bepreferred to enhance optical or mechanical properties of the material.

The porosity of these layers can be tuned from 25 to 50%. In someembodiments, it ranges from 35 to 55%. The synthesis conditions forachieving a porosity up to 50% are generally determined by the solsynthesis conditions. As a rule, porosity of pure MgF₂ layers can beenhanced by addition of aforementioned additives during the solsynthesis.

Different to porous SiO₂ layers, MgF₂-layers obtained by the process ofthe present invention exhibit high degree of hydrophobicity (the contactangle after the annealing process at 450° C. is above 120°). Thisquality results in markedly low water uptake, which for some embodimentscan be below 10% in 90% humidity, whereas it is above 90% in case of aporous SiO₂-layer.

In some embodiments, AR MgF₂ layers obtained according to the method ofpresent invention have thicknesses ranging from 20 to 600 nm. Someembodiments are mono-layers having thicknesses ranging from 80 to 120nm.

In some embodiments, AR MgF₂ layers obtained according to the method ofpresent invention show optical transmission between 98.5% and 99.8%depending on the content of additive M^(n+). In general, the lower theM^(n+)-content, the higher is the optical transmission. In general, thelower the M^(n+)-content, the higher is the optical transmission. Al-and Li-additives do not impact the optical properties, additives likeSn, Ti or Zr decrease slightly the optical transmittance due to theirhigher refractive indices. In general, the lower the additive content,the higher is the optical transmission.

In some embodiments, AR-MgF₂ layers obtained according to the method ofpresent invention are characterized by high adhesion properties and highmechanical stability, especially scratch resistance according DIN EN13523-4 (pencil test).

In some embodiments, AR-MgF₂ layers obtained according to the method ofpresent invention are characterized by hydrolysis resistance superior tothe corrosion/hydrolysis resistance of SiO₂- and MgF₂ layers due to theextremely low MgF₂ solubility and its high temperature stability.

EXAMPLES

Synthesis of Clear MgF₂-Sols

1) 16.29 g commercial magnesium ethylate (MgOEt₂) were suspended in 450ml isopropanol. Then, 0.715 g MgCl₂ were dissolved therein. To thissuspension, 50 ml methanol containing 6.0 g anhydrous HF (aHF) wereadded under rigorous stirring. After 1 day stirring at room temperature,a clear MgF₂-sol with a concentration of 0.3 mol/l was obtained. Thekinematic viscosity of the sol was 1.1 mm² s⁻¹ and did not change over aperiod of 10 weeks.

2) 16.25 g commercial magnesium ethylate (MgOEt₂) were suspended in 450ml isopropanol. Then, 0.715 g waterfree MgCl₂ were dissolved therein. Tothis suspension, 50 ml ethanol containing 6.3 g anhydrous HF (aHF) wereadded under rigorous stirring. After 1 day stirring at room temperature,a clear MgF₂-sol with a concentration of 0.3 mol/l was obtained. Thekinematic viscosity of the sol was 1.1 mm² s⁻¹ and did not change over aperiod of 10 weeks.

3) 16.29 g commercial magnesium ethylate (MgOEt₂) and 0.83 g calciumchloride were first suspended in 400 ml ethanol. To this solution 50 mlethanol containing 6.0 g aHF were added (molar ration_(HF)/(n_(Ca2+)+n_(Mg2+))=2; n_(Mg2+)/n_(Ca2+)=9/1). A clear sol formedinside only about 30 minutes having a concentration of 0.3 related tothe whole 2+ metal cations and of 5 mol % Ca related to Mg. Thekinematic viscosity of the sol was ca. 1.4 mm² s⁻¹ and remainedunchanged over 6 weeks.

3′) 15.4 g commercial magnesium ethylate and 1.67 g waterfree calciumchloride were first suspended in 450 ml ethanol. To this solution 50 mlethanol containing 6 g aHF were added (molar ration_(HF)/(n_(Ca2+)+n_(Mg2+))=2; n_(Mg2+)/n_(Ca2+)=9/1). A clear sol formedinside only about 4 hours having a concentration of 0.3 related to thewhole M²⁺ cations. The kinematic viscosity of the sol was ca. 1.4 mm²s⁻¹ and remained unchanged over 6 weeks. Following the same synthesisprocedure, MgF₂-sols based on different amount of added CaCl₂ wereobtained by changing the Mg(OC₂H₅)₂ to CaCl₂ ratio but keeping theoverall M²⁺-concentration (M=Mg²⁺+Ca²⁺) and the amount of HF (molar HF/Mratio=2.0) and all the other reaction conditions constant. These resultsare summarized in the table 1.

TABLE 1 In all batches magnesium ethylate and CaCl₂ together gave 0.15mol M²⁺ in 450 ml ethanol. In all batches 6 g HF in 50 ml ethanol wereused (molar HF/Mg ratio = 2.0). Mg(OC₂H₅)₂ to clear viscosity mm²s⁻¹ NoCaCl₂•molar ratio after hours after 6 weeks remarks a 95/5  16 1.3stable viscosity b 90/10 4 1.3 stable viscosity c 80/20 3 1.4 stableviscosity

4) 17.5 g commercial magnesium ethylate and 1.5 g aluminium chloridewere first suspended in 400 ml ethanol. To this solution 50 ml ethanolcontaining 6 g aHF were added. A clear sol formed within about 60minutes exhibiting a concentration of 0.3 mol/l and having 5 mol % Al³⁺related to MgF₂. The kinematic viscosity of the sol was ca. 1.2 mm² s⁻¹and remained unchanged over 6 weeks.

4′) 16.25 g commercial magnesium ethylate and 1 g waterfree aluminiumchloride were first suspended in 400 ml ethanol. To this solution 50 mlethanol containing 6 g aHF were added. A clear sol formed within about60 minutes exhibiting a concentration of 0.3 mol/l and having 5 mol %Al³⁺ related to MgF₂. The kinematic viscosity of the sol was ca. 1.2 mm²s⁻¹ and remained unchanged over 6 weeks.

5) 15.5 g freshly prepared magnesium methylate plus 1.4 g dried MgCl₂were suspended in 450 ml ethanol. To this solution, 50 ml ethanolcontaining 8.0 g anhydrous HF (aHF) were added under rigorous stirring.After 24 hours stirring at room temperature, a clear ethanolic MgF₂-solwith a concentration of 0.4 mol/l was obtained. The kinematic viscosityof the sol was 1.2 mm² s⁻¹ and did not change over a period of 10 weeks.

6) 15.5 g freshly prepared magnesium methylate plus 1.7 g dried CaCl₂were suspended in 450 ml ethanol. To this solution, 50 ml ethanolcontaining 8.0 g anhydrous HF (aHF) were added under rigorous stirring.After 20 hours stirring at room temperature, a clear ethanolic MgF₂-solwith a concentration of 0.36 mol/l with 10% CaF₂ (overall MF₂concentration 0.4) was obtained. The kinematic viscosity of the sol was1.3 mm² s⁻¹ and did not change over a period of 10 weeks.

7) 17.2 g freshly prepared magnesium methylate plus 0.6 g dried LIClwere suspended in 450 ml ethanol. To this solution, 50 ml ethanolcontaining 8.0 g anhydrous HF (aHF) were added under rigorous stirring.After 28 hours stirring at room temperature, a clear ethanolic MgF₂-solwith a concentration of 0.4 mol/l containing 10 mol % LiCl was obtained.The kinematic viscosity of the sol was 1.3 mm² s⁻¹ and did not changeover a period of 10 weeks.

8) 16.25 g commercial magnesium ethylate were suspended in 450 mlethanol. Then, 0.715 g waterfree MgCl₂ were dissolved. To thissuspension, 50 ml ethanol containing 6 g anhydrous HF (aHF) were addedunder rigorous stirring. After 1 day stirring at room temperature, aclear ethanolic MgF₂-sol with a concentration of 0.3 mol/l was obtained.The kinematic viscosity of the sol was 1.1 mm² s⁻¹ and did not changeover a period of 10 weeks. Following the same synthesis procedure,MgF₂-sols based on different amount of added waterfree MgCl₂ wereobtained by changing the Mg(OC₂H₅)₂ to MgCl₂ ratio but keeping theoverall Mg²⁺-concentration and the amount of HF (molar HF/Mg ratio=2.0)and all the other reaction conditions constant. These results aresummarized in the table 2.

TABLE 2 In all batches magnesium ethylate and MgCl₂ together gave 0.15mol Mg²⁺ in 450 ml ethanol. In all batches 6 g HF in 50 ml ethanol wereused (molar HF/Mg ratio = 2.0). Mg(OC₂H₅)₂ to clear viscosity mm²s⁻¹ NoMgCl₂ molar ratio after hours after 6 weeks remarks a 95/5  24 1.4stable viscosity b 90/10 18 1.4 stable viscosity c 80/20 12 1.5 stableviscosity

9) 16.25 g commercial magnesium ethylate were suspended in 450 mlethanol. Then, 1.5 g magnesium chloride hexahydrate, MgCl₂.6H₂O, weredissolved. To this suspension, 50 ml ethanol containing 6 g anhydrous HF(aHF) were added under rigorous stirring. After 15 hours stirring atroom temperature, a clear ethanolic MgF₂-sol with a concentration of 0.3mol/l was obtained. The kinematic viscosity of the sol was 1.2 mm² s⁻¹and did not change over a period of 10 weeks. Following the samesynthesis procedure, MgF₂-sols based on different amount of addedMgCl₂.6H₂O were obtained by changing the Mg(OC₂H₅)₂ to MgCl₂.6H₂O ratiobut keeping the overall Mg²⁺-concentration and the amount of HF (molarHF/Mg ratio=2.0) and all the other reaction conditions constant. Theseresults are summarized in the table 3.

TABLE 3 In all batches magnesium ethylate and MgCl₂•6H₂O together gave0.15 mol Mg²⁺ in 450 ml ethanol. In all batches 6 g HF in 50 ml ethanolwere used (molar HF/Mg ratio = 2.0). Mg(OC₂H₅)₂ to MgCl₂•6H₂O clearviscosity mm²s⁻¹ No molar ratio after hours after 6 weeks remarks a95/5  24 1.4 stable viscosity b 90/10 15 1.3 stable viscosity c 80/20 101.4 stable viscosity

10) 16.25 g commercial magnesium ethylate were suspended in 450 mlethanol. Then, 1.65 g calcium chloride hexahydrate, CaCl₂.6H₂O, weredissolved. To this suspension, 50 ml ethanol containing 6 g anhydrous HF(aHF) were added under rigorous stirring. After 2 hours stirring at roomtemperature, a clear ethanolic MgF₂-sol with a concentration of 0.3mol/l was obtained. The kinematic viscosity of the sol was 1.3 mm² s⁻¹and did not change over a period of 10 weeks. Following the samesynthesis procedure, MgF₂-sols based on different amount of addedCaCl₂.6H₂O were obtained by changing the Mg(OC₂H₅)₂ to CaCl₂.6H₂O ratiobut keeping the overall Mg²⁺-concentration and the amount of HF (molarHF/Mg ratio=2.0) and all the other reaction conditions constant. Theseresults are summarized in the table 4.

TABLE 4 In all batches magnesium ethylate and CaCl₂•6H₂O together gave0.15 mol M²⁺ in 450 ml ethanol. In all batches 6 g HF in 50 ml ethanolwere used (molar HF/Mg ratio = 2.0). Mg(OC₂H₅)₂ to CaCl₂•6H₂O clearviscosity mm²s⁻¹ molar ratio after hours after 6 weeks remarks 95/5  41.2 stable viscosity 90/10 2 1.3 stable viscosity 80/20 1 1.3 stableviscosity

11) 17.1 g commercial magnesium ethylate were first suspended in 400 mlethanol. To this solution 50 ml ethanol containing 6 g aHF were added.Then immediately 1.56 g tetraethoxysilan (TEOS) were added. A clear solformed within about 36 hours exhibiting a concentration of 0.3 mol/l andhaving 5 mol % Si⁴⁺ related to MgF₂. The kinematic viscosity of the solwas ca. 1.5 mm² s⁻¹ and remained unchanged over 6 weeks.

12) 17.1 g commercial magnesium ethylate were first suspended in 400 mlethanol. To this solution 50 ml ethanol containing 6 g aHF were added.Then immediately 2.45 g zirconium-n-propylate (Zr(O^(n)R)₄) were added.A clear sol formed within about 48 hours exhibiting a concentration of0.3 mol/l and having 5 mol % Zr⁴⁺ related to MgF₂. The kinematicviscosity of the sol was ca. 1.5 mm² s⁻¹ and remained unchanged over 6weeks.

13) 17.1 g commercial magnesium ethylate were first suspended in 400 mlethanol. To this solution 50 ml ethanol containing 6 g aHF were added.Then immediately 2.13 g titaniumisopropoxide Ti[OCH(CH₃)₂]₄ were added.A clear sol formed within about 40 hours exhibiting a concentration of0.3 mol/l and having 5 mol % Ti⁴⁺ related to MgF₂. The kinematicviscosity of the sol was ca. 1.4 mm² s⁻¹ and remained unchanged over 6weeks.

14) 16.6 g commercial magnesium ethylate and 0.5 g waterfree calciumchloride and 2.31 g titaniumisopropoxide Ti[OCH(CH₃)₂]₄ were firstsuspended in 450 ml ethanol. To this solution 50 ml ethanol containing 6g aHF were added (molar ratio n_(HF)/(n_(Ca2+)+n_(Mg2+))=2;n_(Mg2+)/n_(Ca2+)=97/3). A clear sol formed inside of 18 hours having aconcentration of 0.3 related to the whole M²⁺ cations. The content ofTi[OCH(CH₃)₂]₄ related to the overall MF₂-content was 5 mol %. Thekinematic viscosity of the sol was ca. 1.3 mm² s⁻¹ and remained stableover 6 weeks.

15) 16.6 g commercial magnesium ethylate and 0.5 g waterfree calciumchloride and 2.45 g zirconium-n-propylate (Zr(O^(n)R)₄) were firstsuspended in 450 ml ethanol. To this solution 50 ml ethanol containing 6g aHF were added (molar ratio n_(HF)/(n_(Ca2+)+n_(Mg2+))=2;n_(Mg2+)/n_(Ca2+)=97/3). A clear sol formed inside of 26 hours having aconcentration of 0.3 related to the whole M²⁺ cations. The content ofZr(O^(n)R)₄ related to the overall MF₂-content was 5 mol %. Thekinematic viscosity of the sol was ca. 1.5 mm² s⁻¹ and remained stableover 6 weeks.

16) 16.6 g commercial magnesium ethylate and 0.5 g waterfree calciumchloride and 1.56 g tetraethoxysilan (TEOS) were first suspended in 450ml ethanol. To this solution 50 ml ethanol containing 6 g aHF were added(molar ratio n_(HF)/(n_(Ca2+)+n_(Mg2+))=2; n_(Mg2+)/n_(Ca2+)=97/3). Aclear sol formed inside of 12 hours having a concentration of 0.3related to the whole M²⁺ cations. The content of TEOS related to theoverall MF₂-content was 5 mol %. The kinematic viscosity of the sol wasca. 1.5 mm² s⁻¹ and remained stable over 6 weeks.

17) 16.6 g commercial magnesium ethylate and 0.5 g waterfree calciumchloride and 1.53 g aluminium isopropoxide Al[OCH(CH₃)₂]₃ were firstsuspended in 450 ml ethanol. To this solution 50 ml ethanol containing 6g aHF were added (molar ratio n_(HF)/(n_(Ca2+)+n_(Mg2+))=2;n_(Mg2+)/n_(Ca2+)=97/3). A clear sol formed inside of 26 hours having aconcentration of 0.3 related to the whole M²⁺ cations. The content ofAl[OCH(CH₃)₂]₃ related to the overall MF₂-content was 5 mol %. Thekinematic viscosity of the sol was ca. 1.4 mm² s⁻¹ and remained stableover 6 weeks.

18) 16.6 g commercial magnesium ethylate and 0.5 g waterfree calciumchloride and 1.53 g aluminium isopropoxide Al[OCH(CH₃)₂]₃ were firstsuspended in 450 ml ethanol. To this solution 50 ml ethanol containing6.23 g aHF were added (molar ratio n_(HF)/(n_(Ca2+)+n_(Mg2+))=2; andn_(HF)/n_(Al3+)=3/1, n_(Mg2+)/n_(Ca2+)=97/3). A clear sol formed insideof 20 hours having a concentration of 0.3 related to the whole M²⁺cations. The content of Al[OCH(CH₃)₂]₃ related to the overallMF₂-content was 5 mol %. The kinematic viscosity of the sol was ca. 1.3mm² s⁻¹ and remained stable over 6 weeks.

19) Four different reactions systems were prepared cosniting of 16.29 gcommercial magnesium ethylate that was suspended in 450 ml ethanol each.Then, to these 4 suspensions MgCl₂.6H₂O was given (dissolved): a) 6.26g, b) 1.566 g, c) 2.191 g, and c) 3.13 g. To this suspension, 50 mlethanol containing a) 5.83 g, b) 6.00 g, c) 6.12 g, and d) 6.29 ganhydrous HF (aHF) were added under rigorous stirring. After 1 daystirring at room temperature, a clear ethanolic MgF₂-sol with aconcentration of 0.3 mol/l was obtained. The kinematic viscosity of thesol was 1.1 mm² s⁻¹ and did not change over a period of 10 weeks.

MgF₂-Sols without Addition of an Additive

A) 17.2 g freshly prepared magnesium methylate were suspended in 450 mlethanol. To this suspension, 50 ml ethanol containing 8.0 g anhydrous HF(aHF) were added under rigorous stirring. No clear sol was obtainedinstead a turbid non-transparent suspension of large MgF₂ particles wasformed which did not clear up even after several weeks stirring.

B) 22.8 g commercial magnesium ethylate were suspended in 450 mlmethanol. To this suspension, 50 ml methanol containing 8.0 g anhydrousHF (aHF) were added under rigorous stirring. No clear sol was obtainedinstead a non-transparent suspension of large MgF₂ particles was formedwhich did not clear up even after several weeks stirring.

C) 17.1 g commercial magnesium ethylate were suspended in 450 mlethanol. To this suspension, 50 ml ethanol containing 6.0 g anhydrous HF(aHF) were added under rigorous stirring. No clear sol was obtainedinstead a non-transparent suspension of large MgF₂ particles was formedwhich did not clear up even after several weeks stirring and did notgive transparent coatings on glass.

Formation of MgF₂-AR-Layers on Glass Substrates

The general procedure of producing AR-layers based on sols obtainedaccording the procedures described under 1) to 4) followed the followinggeneral protocol. Optiwhite glass substrates (Pilkington, Gelsenkirchen,Germany) of 100×150 mm were dip-coated with the respective MgF₂-sol.Before dip coating the substrates were cleaned with an alkaline cleaningsolution and finally neutralized by washing with de-ionized water. Aftera drying step at 80° C. for 10 min all samples were finally annealed at450° C. for 15 min. For this, the samples were heated by 5° C. perminute up to 450°, kept there for 15 min and then with the same heatingprogram cooled down.

Formation of AR-Layers on Polymer Surfaces

The durability of the MgF₂ sols obtained according the procedures 1) to4) was performed by dip-coating technique. Plates/foils of severalpolymers (polyethylene terephthalate (PET), fluorinated ethylenepolypropylene (FEP), polyethersulfone (PES), polycarbonate (PC),ethylene tetrafluoroethylene (ETFE), polymethylmethacrylate (PMMA), PCLexan (amorphous polycarbonate polymer), Zeonex (cyclic olefin polymer,CAS No 26007-43-2), Makrolon (polycarbonate)) were generally firstcleaned by treating them with different organic solvents and then eitherdirectly coated without any further treatment or Corona pre-treated orin some cases, a mediator layer made from either Ormosil^(@) orSilazanes was first deposited in order to improve the graftingproperties.

Characterization of the MgF₂-Sols

The hydrodynamic diameter of the nano particles was determined bydynamic light scattering (DLS) measurements using a Zetasizer Nano ZS(Malvern Instruments, Worcestershire, UK) using quartz cuvettes flushedwith an argon atmosphere. The viscosity was determined simultaneous toDLS measurements with a microviscometer from Anton Paar (AMVn; Graz,Austria) at 25° C. Hydrodynamic diameter were calculated bydeconvolution of the correlation functions into exponential functionsusing non-negatively constrained least squares (NNLS) fitting algorithmas implemented in the Malvern Nanosizer software. The zeta potential wasdetermined from the electrophoretic mobilities of the particles in thesol using the Smoluchowski relation.

Characterization of the MgF₂-AR-Layers

The refractive indices and the optical transmission and reflectance,respectively, were determined by ellipsometric measurements with avariable angle UV-Vis spectroscopic ellipsometer SE850 of the companySENTECH Instruments GmbH in the wavelength range between 350 nm and 1000nm. For the evaluation and fitting of the refraction indices n and theabsorption k were used the data set in the visible range (350-800 nm)using the CAUCHY model. The reported refractive indices were taken at awavelength of 589 nm.

The mechanical stability of the AR-layers was tested according thepencil test using a Linartester 249 from ERICHSEN GmbH & Co KG, Germany.In this test, pencil leads of increasing hardness values are forcedagainst a coated surface in a precisely defined manner until one leadmars the surface. Surface hardness is defined by the hardest pencilgrade which just fails to mar the painted surface.

¹⁹F MAS NMR spectra were recorded on a Bruker Avance 400. The chemicalshifts of the nuclei are given with respect to CFCl₃ for ¹⁹F.

XRD measurements were performed with the FPM7 equipment (Rich. Seiffert& Co., Freiberg) with Cu Kα (Cu Kα1.2, λ=1.542 Å) radiation (2⊖ range:5°≤2⊖≤64°).

All the sols described under 1) to 18) gave AR-layers which do notdiffer remarkably from each other based on simple eye inspection. Thecharacteristic data of the MgF₂—Ar-layers made are summarized in Table5.

TABLE 5 summarizes the main characteristics of Ar-layers on Optiwhiteglass Layer thickness Refractive sindex Sample [nm] n₅₅₀ Pencil-Test (1)117 1.2747 6 H (3) 121 1.24 6 H-7 H (3′a) 125 1.27 7 H (3*b) 120 1.29 9H (3′c) 121 1.31 9 H (4) 107 1.27 3 H-4 H (4′) 107 1.27 3 H-4 H 8a 1161.26 3 H-4 h 8b 123 1.25 4 H-5 H 8c 119 1.26 4 H-5 H 11 121 1.24 6 H-7 H12 110 1.28 3 H 13 109 1.29 4 H 14 105 1.26 9 H 15 112 1.27 4 H 16 1091.25 6 H 17 110 1.24 8 H 18 112 124 8 H

I claim:
 1. A magnesium fluoride sol solution comprising: an amount ofMgF2 particles and an amount of additive particles characterized by ageneral formula MF_(m)B_(x-m), wherein M is selected from the groupconsisting of Li⁺, Ca²⁺, Sr²⁺, Ba²⁺, Zn²⁺, Al³⁺, Si⁴⁺, Zr⁴⁺, Sn⁴⁺, Sb³⁺,Sb⁵⁺, and Ti⁴⁺; B is an anionic ligand; x is equal to the oxidationstate of the metal M; and m is equal to or smaller than the oxidationstate of the metal M, and wherein said additive particles MF_(m)B_(x-m)are characterized in that m is x or m is 0 or 0<m<x and wherein saidamount of said additive particles, in relation to the amount of MgF₂particles, is 1:100 to 1:5, as measured in molar equivalent of additiveto magnesium fluoride derived from the first magnesium precursor.
 2. Themagnesium fluoride sol solution of claim 1, wherein B is selected fromthe group consisting of salts of strong, volatile acids of chloride,bromide, iodide, nitrate or triflate, and their partially fluorinatedspecies.
 3. The magnesium fluoride sol solution of claim 1, wherein thesol solution is characterized by the presence of HCl, HBr, HI or HNO₃.4. The magnesium fluoride sol solution according of claim 1, whereinsaid sol solution comprises MgF₂ particles, which are smaller than 20nm, and additive particles, which are smaller than 50 nm in diameter;and/or double salts of MgF₂ particles and additive particles, which aresmaller than 50 nm in diameter.
 5. The magnesium fluoride sol solutionof claim 1, wherein said sol solution has a magnesium content of 0.2mol/l to 1 mol/l and/or wherein said sol solution is stable at roomtemperature for more than six weeks.
 6. A method for coating a surface,the method comprising: providing the magnesium fluoride sol solution ofclaim 1; providing a magnesium alkoxide in a non-aqueous solvent in afirst volume and adding, in a second volume, 1.85 to 2.05 molarequivalents of anhydrous hydrogen fluoride (HF) to said magnesiumalkoxide, wherein the reaction proceeds in the presence of a secondmagnesium fluoride precursor selected from the group consisting of saltsof strong, volatile acids of chloride, bromide, iodide, nitrate andtriflate of magnesium, and/or at least one additive non-magnesiumfluoride precursor selected from the group consisting of salts ofstrong, volatile acids of chloride, bromide, iodide, and nitrate ortriflate of lithium, antimony, tin, calcium, strontium, barium,aluminium, silicium, zirconium, titanium or zinc; contacting a surfacewith said magnesium fluoride sol solution; drying said surface; andexposing said surface in a first thermal step to a first temperatureranging from 15° C. to 480° C.
 7. The method of claim 6, wherein themagnesium fluoride sol solution comprises an amount of additiveparticles selected from the group consisting of CaF₂, SrF₂, AlF₃, SiF₄,ZrF₄, TiF₄, and ZnF₂, said amount being 1:5 to 1:100, as measured inmolar equivalent of additive to magnesium.